My initial response was that it's a horrible shakedown, but on reflection, I wonder if it really was just Saliterman acting in both Everytown's and his own best interest. If it's a common tactic for the NRA to advertise where possible during anti-gun media, and if it's something Snapchat does not involve itself in mediating for journalistic purposes, then it's entirely possible the team that offered Everytown the free media deal inadvertently set them up to have the NRA run messages during their event, and Saliterman was trying to come across as "hey, I would have advised you against this for this exact reason, as the NRA is trying to get in on your ad spots and that's not something we can really prevent."
How you interpret it is probably highly dependent on your opinions of the companies involved as well as how it is presented here. I only wonder because it seems like it's in vogue to call out bad behavior of tech companies right now, and that undoubtedly leads to at least some articles that purposefully misinterpret events to convey that story.
To be clear, this story certainly makes it sound like a shakedown, and it very well may be. I have no idea if that's what happened here, and I doubt at this point we'll ever know know that it's in the realm of PR spin for damage control, but given that both parties refused to comment, I think chances are high that we're getting the author's assumptions rather than something reasonably close to the truth, whatever that may be.