It is necessary to understand where that rule (16) is coming from. It's the anti-gleam-rule from half a decade ago.
I'll quote cg to show rule 16's original intention.

The new guideline has been added to help reduce advertising in the community. Most of our users have seen links to Gleam and related sites, asking people to perform a wide variety of actions to win a game. Occasionally these sites are used by developers to promote their games, but more frequently we see them used by individuals asking others to like Facebook pages, follow Twitter accounts, and click referral links to receive a random Steam key from a bundle. We don't feel our community should be used as a platform to advertise and promote these pages, and for that reason we ask users not to share this type of content in the future.

With the core problem being the amount of actions required to do (and secondary that at the peak of SGs popularity cg didn't want others to piggyback his site for their own).
Since then the guidelines have been (entirely) revised and were changed to be more permissible in regards to general advertising (with activity on SG plummeting before).

Back to this case, as much as I dislike discord, it is basically a space like a website where a giveaway can be hosted. Now if joining that giveaway would require "people to perform a wide variety of actions" then indeed it would not be allowed. The pure existence of a giveaway there however should be fine.

Regardless of any of the above. Linking to just the discord is most certainly allowed as clearly stated.

you are welcome to advertise (...) social media channels (...) or other giveaways.