There have been other discussions about this e.g.
but usually the ask has been the other way around - i.e. if I’m waiting for an automatch and a suitable custom challenge is made by someone then I should be matched with it.
I think the point is that you can set criteria for automatch so can avoid “wacky” settings if you like (although I don’t think anything in automatch is what one might call wacky) and obviously custom games are very specific but as stone defender says, people just don’t read necessarily.
Documentation is here and a wiki
but yes it’s not specific on the lack of interaction between the various ways of starting a game.