That's a fair point relating to other teams, but simply those other teams didn't peak then and got a month of prep instead. C9 and Kru both did offseason events to get ready, then got group since they didn't peak at the right time. Now, after a month break, they're playoff teams.
As for the 2023 examples, look at the game density. Fnatic played their first 13 games (5 lock/in and 8 regional games) in a span of 82 days. Sentinels played their first 13 games (7 regional and 6 masters) in a span of 36 days. Fnatic got 23 days between their lock/in win and their first regional game. Sen got 11 days between Kickoff and Masters, and 14 between winning masters their first regional game. It has been 71 days since the season started and Sen have played the same amount of games that was required for fnatic to win lock/in and get to the upper bracket finals of tokyo (which fnatic got 114 days to do). GenG had an easier time since they got more time between Madrid and the end of kickoff, but they still looked shakey the first two weeks of regional play. So, the madrid teams have had to face a lot more this season than Loud and Fnatic had to face last season. I think it's valid to talk about burnout for these guys.
Edit: Clairifing the grinding point: It's more that the teams who had been grinding were able to hit their stride at the right time, and I'm jusf assuming the teams that looked worse in their ideas and fundamentals were just grinding to a lower degree. It's mainly an assumption, but I think if you drop that point there is still enough there to see that the madrid teams are overworked.