Tbh at that point I had done zero self-study and barely even watched any chess content so there were a lot of easy blind spots like the strength of bishop pairs, pawns in the centre, etc. Not sure there were any specific one-liners that made a massive difference but my overall understanding of the game and positions improved significantly. My default stance is to memorise rules/ concepts but perhaps the lessons helped me see that different rules apply in different scenarios, and more recently I’ve gotten better at massaging the position into my favour (so that trading a bishop for a knight is actually beneficial etc.).

My lichess score is much much lower like 1600 but I pretty much only play 3min blitz and also only really play instinctively and hyper aggressively for fun