Dollar-cost averaging works the same way with any type of investments. Not just Bitcoin. It means that you are investing a fixed amount of money (dollars) at regular intervals.
And what if what you’re investing or the amount of money you’re investing isn’t in dollars but either another fiat currency or in the case of bitcoin or something that totally have nothing to do with Dollars.
The OP has asked a very thoughtful question. Would you still call it DCA in this case?
Well, as already explained, in the case of Bitcoin, you would still refer to it as DCA (Dollar-Cost Averaging) because you are investing dollars (fiat currency) to buy Bitcoin. For other fiat currencies, you could technically change the term to more accurately reflect the currency being used (for example, Euro-Cost Averaging). However, I think thats a pointless nitpick. The US dollar is still the leading global reserve currency, and some phrases are used internationally for the sake of simplicity and universal comprehension. I believe "DCA" is understood worldwide, even when other currencies are involved.