No that's incorrect. We're not talking here about what exactly makes OP great as there are numerous things that makes this series so great and what we would think to be the most important factor, now that would be subjective. If I say the artwork and he says character development, well then it revolves around personal preferences and those are inherent subjective.
However we are talking here about what is the core of the story, where it all is centered around. Strictly speaking you could drag the whole subjectivism/objectivism argument in here, but that would be like saying that Real Madrid can't play football. It's a subjective opinion, but seriously truthfully they are one of the best teams in the world. You can easily test this as if you remove the core, there is no story. Remove the stuff related to that episode and you still very well have a story. If that episode would be the center of the entire plot, then you need to explain how it took 277 episodes just to get an extremely vague idea and that in the following 200 episodes there was barely any progression. That's essentially admitting that OP consists for 95% out of filler chapters, which is not the case. That episode gives another layer to the story, but it's not center.
Putting aside the whole "core" business, I just think it's misleading. It reminds me of the movie Suicide Squad where the Joker gets massively promoted and then he barely had any scenes or even relevance to the story. Why was he even in there? Many viewers felt duped and some of them undertook legal action. It's the same thing here as that episode is not what I would describe as being a good representative of the OP plot as a whole. It's a flash-back where most of the characters only appear in that flash-back or are side characters in the present, there's only one main character and that one only became a MC after a hundred episodes.
Even if people would be intrigued about it (which is already hard as without prior knowledge it's kinda hard to understand that episode), it would take more than a 100 episodes before the topics that are brought up in that episode are touched for the first time.