It's a little fuzzy because the Advice Memo asserts up front that it's assuming arguendo that the memo, in both its protected and unprotected components, constituted protected concerted action. So it's possible that NLRB never really reached the question of whether this kind of organizing truly constitutes organizing for "mutual aid and protection" under the definitions of the NLRA; it didn't have to, because whether or not you're organizing under the NLRA, the memo says there's pretty extensive case history saying you can't do it to work against federal anti-discrimination law.
The NLRA does not eliminate employment at-will! You can get fired for all sorts of dumb superficial reasons. You just can't be fired for exercising your specific rights to concerted action for improved workplace conditions under the NLRA.
But before you go flex this particular right --- and really people should start doing this, like, a lot --- go talk to a labor lawyer and dot your 't's.
This turns out to be pretty easy to do: you can just contact @michelleimiller at Coworker.org, who facilitates this kind of stuff.