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.