I think Discourse’s meta site provides an interesting example.
We see topics all the time about how to move from other platforms to DIscourse, and we also see topics about how to move from Discourse to other platforms, or how to set up an environment that spans multiple platforms.
I don’t see Discourse being worried about this, they know they’ve got a good platform, but they also know it can’t do everything or be everything everybody needs.
That is not even close to same thing. OP had similar situation if someone would ask here how should embed an instagram-video and answer would be link to other forum platform.
I can think of a few CMS platform forums that are hostile towards posts extolling the virtues of other CMS platforms. This, IMHO, isn’t one of them.
Exactly! Thank you! 
I would draw the line between what is a reference link in a footnote to blatant spam/advertising, as in this post you quoted seems ok to me except for the statement
“Register and introduce yourself (at the other forum), then post your problem there”
I would group a statement like that in the same category something like “Buy now, limited time offer 25% discount for the first thirty people to use this link!”
However, if they had simply posted a link to a different forum, that was itself not a really bad forum anyway, with a statement like “You may find the answers you seek at this other site,” I would tolerate a post like that myself but that’s just me. You or any other forum admin would still have every right to take that down or terminate a user posting like this on your forum if this was out of lines with your own community guidelines.