That's only to write to mostr, the mostr posts are carried on most of the major relays.
Discussion
I don’t think that’s true, unless more have added it since I was on Nostr. It was only on a few of them at first.
There are only 3 write relays but it's read on the following which are default relays for both damus and amethyst (as well as others):
relay.damus.io
relay.snort.social
brb.io