I’m surprised how many people are calling warlock a composite when it functions so distinctly from any other class. Literally, it’s the only class with its own, special spellcasting rules that don’t work like any other class’s spellcasting. Even the artificer is more like a wizard than the warlock is.
It's because of (a) the thematic origin being so clearly distinct from other things, due to the explicit existence of a Patron which could (at least in theory) "pull the plug" on the powers, as Cleric/Deity interactions worked in 3e; and (b) the Pact system making it so
overtly obvious that Some Warlocks Are Specifically For Melee By Design, which screams "this is a Fighter/Wizard composite masquerading as its own class."
Now, keep in mind, I'm not particularly taken with the idea of "composite" classes in the first place (I vastly prefer the 4e Source/Role dichotomy for designing and analyzing classes, and "composite" rather loses its meaning in that paradigm.) However,
in the spirit of the question asked, Warlock pretty much HAS to be a composite of
something. It doesn't have the long-standing history to count as its own totally unique thing thematically (even though its expression of that theme is mechanically unique). Then, the significant differences in mechanical expression due to the different Pacts (and, with Hexblade, Patrons as well) cause most people to see in it an inherent composite-ness due to its role flexibility.
Whether this is fair or appropriate is massively a matter of debate. If we rigorously followed your description, for example, then there would be no composite classes at all: Sorcerers can't be Wizards because they have metamagic and Wizards don't. Druids can't be Clerics because they have Wild Shape and Clerics don't. Etc., etc. Every class has unique mechanics, therefore there are no composite classes at all. If we step back from "does it have
any unique mechanics" to "does it have
enough, sufficiently-distinct unique mechanics," well, that becomes a huge judgement call. Is Warlock pact magic a world of difference from Wizard spellcasting because it's based on short rests and always upcasts (for spells below 6th level)? Or is it fundamentally identical to Wizard spellcasting, because the net result is that you cast spells from whatever list you have access to, and a spell cast by a Warlock will be identical to the same spell (upcast as needed) cast by a Wizard? That's a pure judgment call, and I'm sure there are quality arguments in favor of either direction.