You've already consulted the OCD3 and that is even more recent. The traditional view about the existence four classes in Solon's scheme is solidly based on ancient sources. All classes but the pentakosiomedimnoi may well have preexisted Solon, but without the precise definition he gave to membership. There is also general agreement that the treasurers had to be of the Pentakos. It's unlikely that views about these matters will change unless someone finds the relevant part of a now lost Atthidographer.
Some gross errors do find their way into textbooks.
Publishers do not have material conscientiously reviewed. Their simple trust in those who write for them is touching, if one could believe in it. I spend a good deal of time correcting mistakes in books of assigned reeadings, and I doubt I am alone in this. I could mention a commentator on Plato's Euthyphro who imagines that Cronos killed Uranus and Zeus then killed Cronos.