But what allowed NIN and especially SAM to receive potency buffs while SMN received basically nothing in 5.08?

You could argue for NIN buffs, but there is no reason SAM needed potency buffs.

If they received those buffs to "tide them over" until the rework of 5.1, then why didn't SMN receive them too? The difference between MNK and SAM is non-existent compared to the difference between BLM and SMN.