I've noticed that, but personally I thought it might be fair use of referral link however in rule number 4 it's clearly stated that "No referral code (ref link) spam." that's why I want to know that something like that also fall into ref link spam or it doesn't?
4. No referral code (ref link) spam. [1]
This is a fair use of referral link just has you have stated. I rule no 4. refers more to random use of referral link to spam threads and all. But for a signature thread the link of the project that wants to be promoted on the forum must be shared and it's not out of place for the manager to include their own referral code, bonus just have to be earned and in the case of sig thread the managers deserves their credit.