Good luck. Try and apply it correctly. It's correct in the handbook.
And no - legal moves are legal moves whether they're in a game of chess or not, played out of turn, after the game compltes - whatever. They're defined in section 3 merely as mappings from one diagram to another.
What I'm saying is the players can't checkmate with any sequence of legal moves that exceeds the 75 move threshold. That's the criterion in 5.2.2. Similarly they can't checkmate with any sequence of legal moves that are not played in turn.
5.2.2 does not mention a 75 move threshold. It simply refers to checkmate being impossible by any sequence of legal moves. The only way to reconcile the current wording with a 75 move threshold is by denying legality to any moves beyond move 75. Of course, then you run into the issue of weak arbiters wanting to rule a loss against a player that makes three more moves (plies 151, 153 and 155) citing three illegal moves as the justification for the loss. (over the decades I've seen a lot of weak TDs/arbiters that tie themselves in knots over tortuous interpretations of rules used in extremely unusual situations).
As an arbiter that has worked multiple tournaments awarding norms (including continental championships) and signed off on multiple norms, I have no qualms implementing my interpretation (allowing flags or resignations during the last few moves before the 75-move threshold is reached). That said, I will bounce it off of other (and higher-titled) arbiters when I see them next month.
After seeing higher ranked (in FIDE) arbiters, a looming 75-move rule situation does not, in and of itself, trigger a dead position.
I hate people who hate this rule. Seriously, if you can't handle it, move on to another game. Mic drop!
Generally it is a bad idea to make war with rules. But rules are born from concepts which determine the character of the game or sport. For instance, the central figure in chess is the king. It would be out-of-character to buy the revival of your king by donating material to your opponents army. Another one. Chess is a two player game. It would be out-of character to allow a coach to shout hints from the sidelines, whereas this is totally acceptable in soccer. Different concepts. People may always argue about the concepts of a game and that may lead to rule changes in the long run. A well known example is "how long swimmers may stay under water before emerging in a formal competition". How much is swimming under water out-of-character and for which swimming styles? Which has led to rule changes over time.