Alright, let’s use your own words to prove you wrong:
If you start the season with equal MMR as someone, and that someone plays 200 games in the course of 24 days, while you play none, and assuming a winrate over 50%, it’s a mathematical fact that the other person’s MMR has gone up (faster than it could go down, as well) while yours has stayed the same.
Thus, you’re not playing against the same MMR people as at the start of the season, because their MMR has gone up, while yours hasn’t.