That's not exactly true though, your node and the peer you anonymously connect to can both be clearnet for Tor. You just need a tor proxy available to your node and it can route it through an exit nod...
Right but propagating exclusively via Tor/I2P even via proxy is not desirable for everyone due to reliability, but again its def an improvement. I don't think an anonymity set assumption makes sense for Dandelion++ as we're not hiding in a crowd, it's rather the idea of a sneakernet: since P2P traffic is encrypted now it's more the idea of passing messages out of band. You could likely observe who is running a stempool, but shouldn't be able to see much more beyond that