The association is without merit. It wasn't a key factor. It's literally relevance, not advertisement. There's a vast difference. It's not stupid whatsoever. It's the most recognizable character you can get right then and there. Which is going to make Smash itself sell better or the character sell better. Being in the spotlight is extremely important.
Greninja is the exact same thing as Cloud. Both were added because they were interesting representatives of their franchises. Neither were added to promote their latest game at all. That's trying to force a reason instead of looking at why it happened. You don't promote a game that's literally not coming out for a long time or is already out by the time of the Smash game release. Neither were relevant reasons to bother. Cloud was relevant as of KH anyway. Which is my my other point; every 3rd party so far added to Smash was relevant at the time of being added. Which is actually more of why Luminary is the face, not Erdrick or anyone else. Because he's the most relevant Hero to use.
Cloud is an example of Greninja. They were both chosen as relevant characters among the franchise then and there. They were both notable for their own reasons. Greninja had the moveset Sakurai wanted. Cloud was the de facto Final Fantasy character to add. Both were heavily relevant among their franchises, which is a big reason why they were able to get in as well. Sakurai honestly doesn't care that much about whether or not the character will be super popular when adding them, honestly(that is, for newer characters). He's well aware that Greninja could go either way. That's not a relevant factor either. Either the character was plausible due to moveset potential, or they weren't. And in this case, they were. Being a Pokemon, that was instantly going to do fine enough for a base character. All the base characters have relevancy via their franchise when they were added, so it's not like it's honestly as risky as people try to say it is. DLC is when characters become risky. The only time where you have to think more carefully of, since they have to sell on their own merits.
Cloud wasn't promotional at all. Not sure where you're getting the idea he was trying to promote FF as a whole or the remake. He literally is nothing but classic FFVII, Dissidia, and Advent Children, none of the 3 games which were getting remotely promoted at the time. No, the remake is not the same as the original. Again, the reasons you're thinking of don't actually exist. You're mistaking the idea of "being the best representative of a franchise" with "being there to promote the franchise". Cloud is not an example of remotely promoting the franchise as is. The only thing he, again, promotes, at best, is the original PSX Final Fantasy VII game, a bit of Dissidia, and some of Advent Children. The closest thing to anything else is the Chocobo Hat(which had nothing to do with FF as a whole, and more was billed under FFVII and was also using a spin-off game for its design) and the Geno costume(which obviously has nothing to do with FF either).
Greninja wasn't promoting anything either. He was just the best new character that showed up and was plausible within the relevancy window. If he didn't get chosen, there wasn't really other choices either. There's literally zero evidence other Pokemon were fully on the table as is. Unlike with Incineroar, who got in over Decidueye. The only other option. Again, these aren't "promotional picks" in the way you think they are. They're relevancy-based picks. Sure, they technically are promotion-like, but that's no different from Mario being promotional-like for the Super Mario games. Or Banjo being promotional-like for the latest merchandise and the current Rare Replay. That's how unique Greninja and Cloud's promotional usage was. That is, not really there to promote a specific game whatsoever. It's just good timing the remake was announced before Cloud was, which at best put him more in the spotlight.
On the other hand, Luminary might actually be like Corrin and Roy, chosen as the face because it's the latest game in the franchise and to directly promote it. The actual timing of the Nintendo direct implies that's why he's the face. It doesn't mean it's the exact reason. Relevancy is probably more the role than promoting a specific game. Though really, the fact Cloud's franchise says "Final Fantasy VII", not referring to the remake, pretty clearly tells you he wasn't there to promote anything but his own game anyway. Luminary absolutely looks like he's there to promote the latest game. But to be frank, I don't see the issue either because it's just not the entire point of the character, it's just a good face to make him sell better.