Heck, how many fanboys of CONTEMPORARY cultures TODAY get it wrong? Never mind fanboys of cultures that were extant a thousand years ago!
I would emphasise more that every real culture contains an immense amount of diversity, and competing, even contradictory ideas and practices. If there's such a thing as a Platonic essence of Japaneseness, it's not obvious what that thing is or how you'd identify it, much less how you'd live by it.
Shiro Kurita was a fanboy for
particular elements of Japanese culture, as they evolved and were interpreted across millennia. Shiro himself was ethnically Japanese, and while, sure, his bushido is different to previous ideas, the bushido practiced by a Japanese infantry officer in WWII was different to bushido as defined and expounded by Inazo Nitobe, which in turn was different to bushido as romanticised by samurai class bureaucrats in the 18th century, which itself is different to the bushido practiced by warriors and lords in the late Sengoku Jidai, which is different to... and so on and so on forever.
Shiro was inspired by classical Japanese culture and by samurai ideals as he understood them. Did he creatively reinterpret them, producing his own unique spin on those ideas and traditions? Of course he did. And his reinterpretation wasn't the product of a sort of pure Yamato spirit, free and undefiled from worldly concerns: it was tempered at every step by pragmatism, by the fact that Shiro was using it to forge an interstellar empire aiming at the conquest of all of known space.
To me, that doesn't make Shiro a joke, and it doesn't mean he 'got it wrong'. What bushido or the samurai have meant to the Japanese has changed across a thousand years of real history. To charitably interpret BattleTech's fictional history... it's also changed over a thousand years of imagined future.
And yes, BattleTech's particular idea of space bushido is heavily influenced by 80s tropes and samurai and ninja being trendy and so on. BattleTech is not culturally neutral itself, and includes a bunch of things just because earlier designers liked them. (e.g. the Scots and the Irish are absolutely everywhere; someone at FASA really liked Celtic culture. A lot of BTech comes off to me as not only Celtophilic but also Anglophilic.) I do think the design goal with the Combine was not to plot out the most plausible possible future for Japanese cultural tropes and symbols. Rather, it was that giant mecha samurai are awesome.
Even so... I mean, giant mecha samurai
are awesome. And the explanation for how they came to exist doesn't strike me as egregiously dumb. Maybe a little dumb, but come on, we're playing a game about giant robots punching each other in the face. This is well within what my suspension of disbelief covers.