At least this confirms that PoS is fiat.
Not a all. PoS is a technology, like PoW is.
Making it mandatory to use this particular crypto would make it fiat. Fiat means "let it be done" (like
fiat lux) and, on the particular topic of currency, it means that a currency have been established as legal by decree.
Any currency that has value from obligation (decree) instead of actual value is fiat. If a government can hardfork at will and not let users decide if they like it or not, this is fiat. Banknote or cryptographic token, this makes not difference. And the exact protocol (PoW, PoS, PoB, PoI, PoT...) makes no difference.
Making it mandatory to use this particular crypto would make it fiat.
This can't be done with PoW. It can only be done with PoS. You can't force anyone to use a PoW because you don't control it. Your little tinpot dictatorship will be outgunned by your superior enemies that will overpower your miners and reverse their transactions. It would be a tactical error to use PoW in an oppressive regime. PoW is for the free man.