The Winklevii sent BTC because they have a buttload of it, bought at a very low price. Their actual cost for that donation is basically the long-term capital gains tax (and maybe not even that if their accountants are creative).
And yes, Trump (or his campaign) have to convert that to fiat at an exchange. (Probably Gemini, owned by the Winklevii.) But the key thing is that with Crypto, they have no way of knowing where the donations are actually coming from. Someone in Saudi Arabia or Russia could be filling out donations forms through a VPN, and list their location as Branson, MO, and there would be no way for anyone to prove it wasn’t really from Branson. (Maybe if they were careless and reused BTC addresses. Some Monero shill will probably reply and make sure everyone knows how Monero is even better at money laundering…)