I may be completely wrong, but listening to people talk about this game reminds me more of bolt-action than Warhammer. In bolt action, you have early, mid, and late war, and there are different times of the war with different tech and tanks. Succession Wars, Clan invasions, and Word of Blake Jihad are different periods of history, each with different Mech and weapons because techs have not yet been invented. Also, I watched a couple of YouTubers, and they have a percentage of how much a faction has said Mech, much like in a history game where they may tell you how much of a weapon was produced and how often it was. So, am I accurate? Is BattleTech more similar to a historical game than a normal fantasy or sci-war game?
I would personally say that BattleTech is more a normal (if significantly above average) wargame, as opposed to Warhammer and its ilk which are more the outliers in many ways (including, let's face it, for neutral or for ill, size and name-recognition).
The rules are explicit about offering abstraction instead of simulation, though. But to my mind, rules crunch doesn't provide simulation all that much, it provides immersion and detail. That always has been the duality that comes with it being a game. But is it military-scifi? I would say yes, in places to a fault, even. 
A simulation is only as accurate as its least accurate component.
Rules crunch absolutely does directly bear on a simulation, but it very much depends on what you want to simulate. BattleTech at its core, doesn't really simulate the real world armoured combat terribly well (non-BA infantry, for one, being essentially chaff until quite late in rules or the fact that 12m-tall giant robots are not mowed down as easy targets by tanks because they can't hide anyway for another), but that is kinda of the point of its main contention (Giant Robots), so that isn't an important factor.
But even that said, BT DOES, however, pay a lot more attention to a lot of important simulationist details most wargames (historical or otherwise) entirely ignore (e.g. dead ground) and makes a very commendable attempt to at least cover and integrate all aspects of warfare, including off-table artillery strikes, air-support, campaigns, logistics... And in a fashion in which is left entirely open (and with increasing choice of what rules to use) for players to use or not. On top of the fact that one can be really keen into the lore and make canon-accurate units, down to the colour schemes and what variants that faction used when... Or completely ignore all of that and just make forces entirely based on What Mech Looks Cool.
(The latter of which I am a card-carry member, because I treat BattleTech "casually" compared to my other wargames (one of which (Manouvre Group) is fundementally a real-world tactics simulator first and foremost.))
