Your OP basically makes the rules flexible. Players and DM can pick and choose which set they want to use.
Another big part of my inference come from this later exchange:
Now, I may be wrong here, but I interpreted this to mean that it's the job of the players to communicate what versions of spells and feats they want to use. But you, as the DM, are under no burden to communicate what you are using.
Combining that with this from the OP:
Putting all this together, it sounds like you're advocating for using 5e as a loose set of rules to start from, basically a tool to give the players a baseline for creating characters. But not as a hard set of rules anyone must follow. Especially not the DM, who is not required to communicate which version of any ruleset they are using. The DM is allowed to not only pick and choose any version of a monster they want, but is free to "make stuff up as they go". From there, my inference is that the DM isn't really holding themselves to any set of rules at all; what's the value of a rule if it changes on a whim with no notice?
Admittedly, going all the way to "I don't track the HP of the BBEG, he falls when it's appropriate for the battle to end" from there is obviously taking the inference to a bit of an extreme. But it's also a completely legitimate playstyle that people advocate for, and seems very in line with the philosophy "I don't really care what rules we're following."
Alternatively, looking at this whole idea from the other end: If you don't care about what rules people are using, what do you care about?