It's a philosophy behind the intention of the product. Since the notion of neutrality is a zone more than a single thin line, how far you push (or don't) within (and outside) that zone are all choices made by the manufacturer based on the target audience. If you're aiming for a studio use, that's different than a hobbyist enjoyment use, especially when slight tweaks can shift the sound in impactful ways. For example, imho the HD560S is best for production and monitoring, the HD660S2 is best for enjoyment. Both are in the zone of neutral, though frequency response is only a part of the reason why they are meant for different audiences. Tbf, some folks (including lot of well-regarded reviewers), love that flat studio tuning but I find that it often puts mids too far forward, and bass is an afterthought. Ymmv.
Just in terms of the MM-100, if I was to use it for mixing, I'd still want to throw on a little EQ, though I'd be a lot more precise about ear gain and such. For enjoyment, I find that a general lower treble pull as I mentioned up thread works for me, and when paired with something as smooth and refined as the RU-7, the MM-100 can sound quite enjoyable.