Holding stock in a company (for instance via en index fund) is not a fiat holding. The fact that it is commonly priced in terms of fiat currency is not any different that the fact that BTC is too.

Neither of them have to be. All exchanges take place in pairs; one holding is exchanged for another. Buying BTC with dollars is (USD, BTC); buying Microsoft is (MSFT, USD); buying EUR with dollars is (EUR, USD).

Nothing prevents you from pricing Microsoft stock in BTC or BTC in EUR or Etherium.

The holding as such, however, is only a fiat holding when it is in fact either a (wad of) cash equivalents and/ or a bank balance.

There isn't a person on earth for which there is only two types of relevant holdings, in either USD or BTC. A person that (seriously) considers acquiring a crypto holding, such as BTC, is most likely choosing between other crypto currencies, gold or silver, primarily. Even more likely, they're merely FOMO-speculating as a complement to plain stock holdings.

The fact that a number of you don't seem to understand this should call your competency into question.