Maxperson
Morkus from Orkus
If it simply doesn't mention the modifications from the 5e MM, then the 5e MM are still the default. This is from the PHB and shows how the 5.5e rules work.Dragonborn NPCs without breath weapons are the default according to the 2024 Monster Manual. Dragonborn NPCs use the same stat blocks as human NPCs, without any of the modifications which were mentioned in the 2014 rules.
Personally, I don't agree with that design choice, but it remains the default rule in 2024.
"But the book contains many new or redesigned elements, and the versions of things in this book replace versions from older books."
Since the 5.5e MM hasn't redesigned NPCs in that regard the way that it has the hide rules and such, the 5e MM rules still apply. Their quest for backwards compatibility is the most likely reason why this is the case.