It's a silly argument really. It inherently acknowledges that their problem is already solved by Nostr, but since they themselves are unwilling to simply use that and onboard people to their own relays and/or clients, the solution is not good enough for them and their peers. Maybe it's a practical joke by an xkcd fan? 🤔

Yeah, so my idea (and rough plan) was for kosmos.org users to have anything they post to the Nostr relay be synced to their storage automatically, so then other apps could go and cross-post content from there (in addition to just having a syncable backup and potential cache there, too). As you say, the transports can be much more agnostic, as long as the original content is signed by someone's key.