Nit picking. New leaders and new modes definitely change the way a game can be played. You can play the game in a totally different way than you could before.

It expands the game with enough material to be considered an expansion. No need for an arbitrary definition of what an expansion is.
Then we should call them other DLCs Expansions too, because some have 2 Leaders, World and Natural Wonders (and even resources) and also have Scenarios that have a unique gameplay.
For me, it's not about how much Content you add (you can add that bimonthly with small DLCs) but it's about the main Features of the expansion and how they
force you to change the way you play the Game. Deciding to play with an NFP Leader isn't any much different than choosing one from Rise and Fall XP for example, and you can have the Game Modes enabled and completely ignore using the features they introduce. Barbarians will still attack you as always, ignoring getting a SS, acquiring Heroes and building Monopolies won't affect much how you usually play the Game, I mean you can say that AI using them adds a little challenge, but that's not the point. Tree Shuffle Mode is a good one though, because it really affects how you play the Game, without it you would always know which route to take and plan ahead, but with it you can't do that, that's a little game changing, but a good one, more of that! Dramatic Ages is just darker dark ages, and Apocalypse just builds on the already existing disasters and takes place too late in the Game. With Rise and Fall on the otherhand, the Loyalty System introduced affected the game from ancient to future era, you need to think more on where to settle, worry about neighbours and conquering cities became a totally different game now. You can't ignore any of that. After all, the Expasions are called after the main Feature they bring in.
I understand why you see it like that, and I'm also glad that you enjoy NFP, but on my part it's far away from being an Expansion. Mainly because you can sell me New Game Features and Mechanisms but not New Content. It's the Game Mechanics that I'm more interested in, the Features that extends the Gameplay and affect how you play the Game (no matter the Leader). I can't get that with a small DLC, so Expansions are my Favorit Shop for that. Did NFP add those Features? Yes. Are they any comparable to regular Expansion Features? not at all, some of them are actually just variations of Features from previous expansions. So if NFP
is an expansion then I expect Civ 7 to have similar Expansions, and if that's the case then I would diffenitely say no to Civ 7 and stick with civ 6 or hope for an improved
Humankind.