I would assume that if a bank claims to have my deposit of say $100, it can prove that it has that money physically in a safe, or more likely a corresponding position in the forex market.
But I see your point. It's very hard to actually prove that your dollars in e.g. eToro or Revolut actually exist.
The question is, when could this become a risk? Maybe in case of a bank/app run? It probably devaluates the dollar too.