I think another way to say this is that MDF works only if you're in a spot where you have hands that are strong enough to call. If you play every hand, and you see every river in that 100into100 situation, you shouldn't call with 50% of your hands because your hand range is too wide for that to be profitable.

So you can't make a ton of mistakes say "MDF" and call off, you have to have done the right things in previous streets to end up with a range that can call at MDF. That range (and those street actions) require an understanding of GTO (and the adjustments needed when someone isn't playing GTO).