The word "fiat" does not mean what you think it means. Magic items by level 8 are not required by the rules, but they are norm for the adventures. They are also the norm for the PC classes. Even the classes which are not spellcaster based usually obtain something in the class or subclasses or otherwise to get past resistance to non-magic attacks by mid levels.
Apes appearing with rocks, however, is not a norm - you polymorph into the creature, not their equipment. They simply don't come equipped with them - it's not listed. And since it's the spellcaster determining when the spell is cast, it's not even up to the DM to supply you with rocks. In no way is this a comparable concept.
And I never said or implied anything about a +2 or +3 weapon. I said it's the norm for PCs to have found something to bypass resistance to non-magic weapons by 8th level. That's it - that's my claim. It's not a weird or out there claim. Polymorphing into a Giant Ape appearing with rocks however is a weird and out there claim.