DF gameplay is quite unique. Fortress mode is definitely close to the RTS genre itself. Army + resource management, real time gameplay. So basically it is a combined rts/city builder.
Agree with most of what you say. If we were to try to define it, DF shares some overlap with the conventional RTS form, though neither resources, production facilities nor troops really adhere to that template. It also has elements of economic simulator, city builder and god game. So DF is absolutely unique in that it is an almost-hybrid of these types that is less than and more than all of these at the same type. So yeah, DF is unique and we can't really subsume it under any existing mould.
Had RTS been named better we would have a lot less terminological and meaningless confusion and in today's computer game theory debates.
I think most "RTS" games would be better named "small army tactics" or "regional resource and combat" games. Nobody would actually use those names, but the problem is that very few RTS games involve anything as grand as strategy, and you rarely command more than a hundred military units.
Zerging is a tactic. Turtling is a tactic with some strategic elements. Economic boom with a future plan for a crippling blitzkrieg is somewhat strategic, but relies on good tactics to work. Choosing to establish a trap-laden fortress of cave-dwelling dragon-slaying luchadores in the one mountain pass that connects your civilization to the goblin civilization is the cornerstone of a (very defensive) strategy. It's like the Maginot Line, only finely decorated with cat leather and hanging rings of goblin bone.
Extra pudding for +Maginot Line+ menacing with spiked of kitten leather
You're bringing up the second of the two huge confusions surrounding "RTS" games: the meaning of "strategy" and "tactics". The problem is that when classifying strategy wargames (the super-genre that include real-time and turn-based strategy- and tactical games [RTS, RTT, TBS, TBT]) these terms are used differently from in dictionary. Whereas military definitions (correctly) refers to long (war) term vs. immediate (battle) term methods, games rather mean strategy as managing economy and production (the focus of typical traditional RTS games like Dune II, Warcraft and Starcraft) and tactics as battlefield manoeuvres (formations, controlling heights, etc; RTT games such as Close Combat, Gettysburg!, Warhammer: Dark Omen, and the battle sequences of Total War).
So yeah, using "correct" dictionary terminology Zerging would be a tactic you can use while the order you build your buildings would be relatively more strategic choices (in context), but
Starcraft is still a RT
S (strategy) game while
Myth II: Soulblighter is a RT
T (tactics) game with the more limited game genre usage of "strategic" and "tactical". Rather silly and poorly named, yeah, but something we have to live with.