The macro code for Norwegian ("no") is used in some places for Norwegian Bokmål instead of the particular code ("nb"), because the Norwegian Bokmål Wikipedia has "no" at its domain. This should be avoided if possible.
I notice I get "from=nb" in the URL when I go to an article on no.wikipedia.org, and click the greyed out "nynorsk" link on an article that has no nn.wikipedia.org translation. And Content Translation then says "no MT available". Manually changing the URL to "from=no", I do get MT.