Like someone before, I'm pretty amused by the "famous secret" sword style.
Other than this, I read the discussion in these comments about how he's quite weak for someone who trained for millions of years.
I agree, but it does make sense in a way. He has no talent, and had no partner and no teacher in the other dimension. He basically had to reinvent swordsmanship from scratch, without a clue whether he was even doing it right.
Also unsure if his physique improved at all given that he obviously came out of this dimension looking exactly the same as before.
Things are a little unclear, but the lack of talent and guidance definitely explains how he didn't turn into a one-punch-man type character.