I think the best way to look at it is as three different phases - Action, Treasure, Buy. With the caveat that all things referring to the Buy phase would then refer to the Treasure or Buy phase.
Yes, I think the rulings, if not the actual rulebook, have clarified that there are sub-phases.
The Action Phase has the "start of turn" sub-phase and the "play your action cards" sub-phase.
The Buy Phase has the "play treasures" sub-phase and the "buy stuff" sub-phase.
The Cleanup Phase has the "discard everything" sub-phase and the "draw a new hand" sub-phase.
If I remember correctly, it was eventually ruled that buying a card from the Black Market with Capitalism in your "play treasures" sub-phase does not prevent you from spending coffers after that; so it's not "buying during your buy phase" that prevents you from playing coffers, but rather moving on from your "play treasures" sub-phase that prevents it.