I'm not really an expert on either so I'm not comfortable giving a decent answer to that but I guess the metadata problem in Nostr DM's doesn't exist on Matrix because matrix server traffic is not public. Matrix is too bulky for DMs in my opinion but it provides excellent privacy and unruggability. It's just not efficient enough in contrast with any alternatives using MLS (many will come I believe). Nostr on the other hand is pretty lightweight but has terrible privacy, similar to Bitcoin itself when used that way. I have no idea how to implement Nostr DMs "the right way".