Lemmy is implemented in Rust using the Diesel ORM/Query Builder for persistence. I don't know enough rust to comment specifically, but based on my knowledge of other ORMs and stuff it should be possible to support different database backends, but it would likely not "just work" without some effort on the part of the developers.
One of the things that is being done for the next release is a bunch of pg optimization work, which to me makes it even less likely for it to work with minimal changes, as they are likely putting in some amount of postgres-specific code to achieve those gains.