Indeed, that thought has crossed my mind. That does not change that it's still an appropriate use of a hashtag.
As an example, I was at the Crystal Bridges museum of American art last week. They had an exhibit about superheroes in pop culture: Men of Steel, Women of Wonder. They had signs saying it was okay to take pictures (without a flash) and to tag them #moswow. Now, by Gas's argument, this is a terrible tag, because no one would search for moswow if they had never seen the sign at the exhibit. By his logic they should have had #Superman #WonderWoman #CrystalBridges #NormanRockwell #EtcEtcEtc on the wall, and not the specific hashtag about the exhibit. Never mind that someone seeing an Instagram post from a friend at that exhibit, could then go on to follow the link in that hashtag to see the posts other people had made from there, without finding all posts about Superman, or all posts about Wonder Woman, or all posts about the museum as a whole. They'd be able to find posts about their specific interest, which is the purpose of hashtags in social media.
Why would a letter have any hashtags in it at all? At least, if it's a letter that can't have hyperlinks. As MD said, it would have been best to not write the hashtag on the paper and put it in the tweet the picture was embedded in (and that's not even touching the inanity of taking a photograph of a handwritten piece of paper and posting that instead of just straight up typing in an electronic medium to begin with). And then it could have had all the properly descriptive tags as needed.
Hashtags don't exist solely for searches with no context. That's like saying "If someone followed this link to page 12 of the webcomic without having read the first 11 pages, it would be completely useless." The purpose of this hashtag is to allow people who have been made aware of the issue to easily find other people discussing the issue. That is part of the explicit purpose of how hashtags are supposed to work. Oh, hey, this tweet has been tagged #SpecificTopic, I can easily click on the hashtag to find other people discussing Specific Topic.
That also breaks down because all too often people just go crazy inventing extra hashtags and drowning useful ones in a sea of nonsense. I'm not railing against Troy and Abed using the hashtag #AnniesMove to livetweet Annie's Move.
Are you wanting to have a discussion with people on social media about, specifically, pale redheads in stockings turning you on? NO? Then what you're saying here has no relevance to the issue at hand. The MCU marketing team wants people to discuss the subject of not spoiling Endgame. They want people to easily be able to find others discussing not spoiling Endgame. Finding other people discussing the #MCU in general, or even #Endgame as a whole would not be the same as trying to find the specific topic of #DontSpoilTheEndgame. They also wouldn't want to use #EndgameSpoilers because that's a tag people wanting to avoid hearing spoilers wouldn't read, whereas they might read a discussion about NOT spoiling the movie.
And really, my initial comment was not specifically about this letter, but about the common forms of hashtag abuse. We seem to have gotten a little off into the weeds, then trying to apply my gripes about the practice in general to this specific example.
But yeah, it's still extra stupid to write a note by hand, put a hashtag on the paper, take a picture of it, and tweet that picture.