It has to be a "chatroom of any particular
groupchat protocol". While
you are free to argue that 1:1 chats are groupchats of a group of two
and therefor regular chat messages form a groupchat protocol, I doubt
it was intended that way by the authors (neither in XEP-0402 nor in any
other XEP).
What makes it still clear in the XEP, is that the bookmark must be of a
*chatroom* of a groupchat protocol, and a regular user JID is certainly
not a chatroom.
I disagree. I think it is a chatroom. Author intention doesn't seem relevant
here, and bookmarking 1:1 chats doesn't violate anything the XEP already
requires an implementation to deal with (namely, that the groupchat may use
a protocol other than MUC).