Lieutenant Colonel Arachnid is a pinnacle Silver Realm powerhouse. But he once engaged in a controlled battle against a captive Red tier predator where he lasted for 5 minutes which was a significant feat.
This was the reason why this Lt. Colonel's name was associated with the fact that his strength almost rivaled Red tier predators.
Of course, this association failed to add in the fact that the Lt. Colonel fought in controlled circumstances and under a set and practiced order.
What this meant was that Lt. Colonel Arachnid never had the strength of a Red tier predator, he only lasted 5 minutes against one because he was in his home ground where he was familiar with the terrain and the order of the battle.
Sebastian just underwent an evolution. He already fought a sizeable number of battles against purple tier predators, he had a good idea about how strong they were but he had never fought against a fully fit Red tier predator.