I'm not clear on your distinction.
It's simple. PLAs auto-Augment and ignore XP Costs among other things but normal Manifesting is supposed to pay for it. Saying Manifesting is
considered a PLA, rather than every Power Manifested
is a PLA, is a way link some of the shared rules without actually saying they are going to be treated or handled the same in every regard.
Similarly PLAs are not the same as SLAs and the text never makes such a claim. What is actually says is Spells, Spell-Like abilities, and Magic Items that could potentially affect Psionics do affect Psionics. In other words, Dispel Magic can affect a Psionic Power, not Dispel Power is a Spell and can be used to enter Spellcasting Prestigious Classes.
Finally, and I'm going to trim the entry here and it sucks Snakeman is here, but.
Benefit: You add Spellcraft to your class skill list, which allows you to attempt to convert an arcane spell into a power you can add to your repertoire. Each spell costs a certain number of power points to manifest. Because the spells are now effectively psionic powers, they are no longer affected by metamagic feats. However, metapsionic feats can affect them as they would a psionic power. As with casting a spell, manifesting a spell may require certain components.
StP Erudites convert Spells into effectively-a-Power which are exempt from Spell-related rules and they very clearly Manifest, not Cast, them. They never gain the ability to cast a spell. Sure they can produce the end result of getting a Black Tentacles on the field, but that's not what Requirements check for.