Well, it could be the idea of a parallel universe or different realms which is somewhat common in both. In Pendragon, there are multiple worlds and in Harry Potter, there is the wizarding world and the Muggle world. The concept of a character being 'chosen' or having a special destiny is also there in both. Harry is the 'Chosen One' and in Pendragon, the main character also has a special role to play. These shared concepts might make people think Harry Potter has elements of Pendragon fanfiction.
Some people might think this because of the common theme of a young hero on a journey. In both, there is a central character who has to face great challenges and grow throughout the story. Also, the use of magic or some form of special powers is present in both. In Pendragon, there are powers related to the different worlds and in Harry Potter, there are spells and magical abilities. This similarity in the basic structure of the story could lead to such a thought.
People might consider Harry Potter as a Pendragon fanfiction due to the similarities in the moral and ethical themes. Both stories often deal with good versus evil, the importance of friendship and courage. Also, the way the hero in each story has to learn and adapt to new situations. Harry has to learn new spells and about the wizarding world, similar to how the hero in Pendragon has to learn about the different worlds and their rules. This resemblance in the overarching themes could be a reason for the thought.
Chapter 331, "Zhu Shun, Huang Tianxi, the Third Cold Yuan Banquet of Great Xia, and Xia Yuzong", is the clearest early answer because it turns Zhu Shun's situation into a concrete plot event. It opens with Inner City's main street, Zhu Shun led his wife Huang Yuniang and... and closes on But later, as the Camp Supply Department sent over the ceremonial....
Chapter 312, "Regaining Focus, Killing, Shock, and Submission (Part 3)", is the clearest early answer because the violence around Regaining Focus becomes final instead of hypothetical. It opens with Those seem to be the families of Meng Ying, Jiang Ping, and several... and closes on With a slight wave of Xia Chuan's hand, Yue Feng and the others....
Chapter 250, "Rescue, Intimidation, and Surprise (Part 2)", is the clearest early answer because it turns Li Hu's situation into a concrete plot event. The chapter turns that into something concrete by moving from Those people in light gold armor have been using solidified fire oil... to That thing can actually eliminate dew, it's amazing!" Among the three....
Chapter 315, "New Policies for Long Mountain, Cold Apple Tree and the Silver Mine", is the clearest early answer because it turns Shi Ping's situation into a concrete plot event. The setup is Regarding the issue of the nobility system, there is still a long..., but the pressure keeps building through Now let's talk about the eight new deployment points in Longyou?" Xia....
Chapter 308, "Shocking Turn, Grave Injuries, and Restless Hearts", is the clearest early answer because it turns White Dew Gu's situation into a concrete plot event. The chapter turns that into something concrete by moving from How is your strength possible..." The moment Wu Lin collided with Xia... to Under such circumstances, Xia Hong was still willing to put effort....
Chapter 340, "Seclusion, Growing Yu Generation, Family Banquet, Rapid Development", is the clearest early answer because it turns Lu Yang's situation into a concrete plot event. Early on, As the night of the tenth day of September descended, Lu Yang set out.... By the end, Though Xu Ning and Lu Yang are not extraordinarily strong, they have....
Chapter 304, "The Defeated Longyou Army, Xia Army Cannot Escape, Holy Tripod", is the clearest early answer because Great Xia gets a payoff that changes the stakes. The chapter turns that into something concrete by moving from The people's expressions, which were originally somewhat dispirited,... to Long Mountain originally had just over 70,000 people..
Chapter 331, "Zhu Shun and Huang Tianxi, Cold Yuan Banquet of Great Xia Year 3, Xia Yuzong (6)", is the clearest early answer because it turns Cold Element Festival's situation into a concrete plot event. It opens with Xia Hong suddenly let out a hearty laugh and raised his glass to... and closes on Wah..." Over a hundred people, all at Cold Resistance Level,.... The key payoff is that the chapter closes with It's not only Xia Yuzong's birth;
Chapter 304, "The Defeated Longyou Army, Xia Army Cannot Escape, Holy Tripod", is the clearest early answer because Xia Chuan gets a payoff that changes the stakes. Early on, Xia Chuan realized this point earlier than the others, so as soon as.... By the end, Following the leader to achieve such fortune, I've lived enough.. Inside the broader exploration storyline, the chapter closes with "Tend to yourself first;
Chapter 306, "Rebel Collaborators, Chaos and Opportunism, Long Mountain Changes Hands", is the clearest early answer because it turns Xia Hong's situation into a concrete plot event. Early on, Xia Hong's expedition to the north never intended to capture Long.... By the end, In other words, without those three thousand rebels, the.... Inside the broader family politics storyline, the chapter closes with "What hope? That Golden Armor Army from Great Xia hasn't moved yet;