It is true that fiat cannot be replaced by Bitcoin, but it is recommended to trade Bitcoin for intra-trade transactions.
Using bitcoin for intra trade transactions can only be achieved when bitcoin has gotten full adoption globally but for now, intra trade transactions won't work except between countries that has accepted and adopted bitcoin as a legal tender such as El Salvador and Central African Republic.
With countries adopting bitcoin as legal tender side-by-side their fiat currency it's definitely going to make all types of trading transaction seamless for everyone. But even with the barriers that may exist from the government of any country towards bitcoin, intra trading transactions can still be successfully processed and achieve among different traders from different locations provided there an already built trust that exists between the traders and they both agree to transact using bitcoin then it's a done deal whether or not bitcoin is a legal tender in that country.
I think the individuals agreeing on using bitcoin for the transaction override whatever the government policies says about bitcoin as everything is going to be done with privacy.