I don’t know exactly how their fedimints work but I currently have some funds in one right now as party of my mutiny app and I am certainly not running a fedimint server
To be clear, the ring sig I suggest here isn’t about the custody of funds, but as a way to announce who is operating the mint without actually revealing specifically who is operating the mint. This could allow mints a new position in the privacy-transparency tradeoff. I don’t believe this has been implemented anywhere, all mints are either privately operated (and may steal your funds) or transparently operated (and the government may steal your funds in a seizure).