I'm not as sure as you are about the Buddha not teaching about levels of liberation.
You find the same descriptions in the northern canon (agamas), so it means it was "invented" before the schism of schools, maybe less than 200 after the Buddha's parinibbana maybe.
What's more, the theory of the levels of liberation fits well with the list of the fetters. What's more, the term, ariya is found everywhere in the canon, even in the definition of what the (true) sangha is.
It can seem odd that the Buddha taught in such a "prize-winning" way, but often it is seen that the Buddha insists on the respect one ought to have for the Noble persons, the true Brahmans etc...in a society where respect and honour was proportonal to the cast you belonged to, it's not improbable that the buddha recast in some way the mentality and put honour and respect where they should belong, i.e, to those with a true attainment.
Finally, if it was invented by monks, I don't know why you have suttas declaring lay people to have reached stream entry etc...
But that's not the debate here. Maybe it's the opportunity for a new thread.