I don't think this compromises the artisticy and authenticity of the recordings in any way.
But when push comes to shove, everyone has his or her own personal "artistry and authenticity" threshold.
There is no objective or independent criteria that sets a point at which editing defeats "artistry and authenticity".
It is entirely a matter of personal judgment, as is any artistic or aesthetic judgment.
Any other perspective smacks of elitism and, at its worst, the kind of state control of "what counts as art" that typifies the worst regimes of this and the last century.
An entirely separate matter is what Piano Society wants to make its mission or purpose. The members of the site have a perfect right to stipulate what kind of material they want to put up, how much editing they are prepared to allow, and whether or not a real instrument recorded by microphones is all that should be permitted. Clever sequencers or anyone else will, as I have said, have no difficulty ignoring such rules, and even those who abide by them in spirit will be tempted to go beyond what is allowed, because there is absolutely no way of policing the rules.
That is the practical problem.
The intellectual or aesthetic problem is that some of the material that is ruled out by theoretically strict (or at least stricter) rules about editing may be of very high artistic value. Not, perhaps, to those (like my parents, for example) for whom only the concert hall is the only really authentic musical experience, but to those who, once they have discovered how much a recording may have been edited, lose interest in it.
But, again, this sort of aesthetic issue boils down to personal preference, at least in my view. I do not think the alternative view, that authenticity and artistry can be objectively measured, is politically or aesthetically defensible--and it has very few if any adherents in the history of aesthetics and philosophy.