When saying DCA does not mean that the coin we are considering is dollar in this regard, but it was just used as a measure to how we could quantify the use and conversion rate of bitcoin in fiat currency, because if you look into it very more closely, DCA is not mostly practiced in fiat, but we use it with the bitcoin network, just as you can have your bitcoin in any rate, but uses dollar to qualify the rates they are worth.
DCA is often referred as using dollar or fiat currency to invest and average your investment entry price. This is often expressed like this because fiat currencies in general and US dollar are most popularly used in societies.
However, generalizing it to an investment strategy, you can use anything as your investment capital to an asset, it can be called as DCA. You can use US dollar, other fiat currencies, gold, stable coins or even altcoins for DCA Bitcoin. But it's more risky to hold stable coins especially altcoins, because they can depeg (stable coins) and lose most of their values (altcoins) so it's not recommended to hold them.
Use US dollar, fiat currencies or gold for your Bitcoin DCA is better and safer in risk management.