In the explosion, due to the Super Base Pattern's absorption of the World's Force increasing to a terrifying level, a series of chain reactions occurred, enhancing Lin Shen's various abilities.
So when the Fallen Seraph made the first strike, Lin Shen felt as if it wasn't as strong as he had imagined, but that was just a guess at the time.
During the second strike, Lin Shen confirmed his speculation; indeed, as he thought, his abilities had all enhanced in the explosion, which is why he felt that Flarela's strength wasn't as formidable as he had anticipated.
This was not an illusion, nor was it Flarela holding back, and certainly not an overestimation of Flarela on Lin Shen's part.
In such a horrific explosion, Lin Shen's Super-Base body was constantly shattered, but then it rapidly recovered, which is why he seemed unharmed in the midst of the Light Explosion.