So to be clear, one would want this to be the only relay they read from? Would that affect seeing other posts?
This has been broken since the day it was launched, sorry if you tried to use it in the meantime.
But now it seems to be working, finally.
And there is an explanation at https://inbox.relays.land/.
Discussion
Yes, the idea (for now) is that this will be the only relay one will read their _mentions_ from (as described by NIP-65). It won't work in clients that do not (yet) operate in that paradigm.