I'd agree that, in the awakened state, samadhi can come on (and persist) in any state of standing, sitting, or walking, and is never scared-away by any actions, movements, activity, or non-actions for the length of time that awakening stably persists (while supported by one's continuing practice: weeks, months).
But in order to awaken (in the first place), samadhi is best encouraged and "hosted" while one is in one of the close sitting postures (variants of Lotus; variants of Seiza).
It's noted in some circles of the Zen tradition (I think... ) that awakening often comes about as one suddenly awakes out of samadhi (as samadhi "breaks-up"). Thus, if one values a possibility to awaken, it can be helpful to prepare the ground, and to enable and to cultivate the samadhi states, and this seems best done in the traditional postures, which have become traditional for this reason, not because it's "just the way that people [who didn't have chairs... ] used to sit" (in my reasoned opinion).
A good package-deal we have, here/there.
So, where does that leave us? Oh, yes; >>-----> cultivate a sitting posture that your teacher recommends which he/she sees is possible for you, in the current state of one's body, and as it can develop through practice and as a result of practice. I.e., don't just automatically "default" to a chair. If you do, 'de fault' may be your own!


--Joe