Yes, because it doesn't have any official statements to back it up, you're treating it as reason for a hard, definite no rather than simply unlikely, and your reasoning based on that point is inconsistent.
Somehow these two:
View attachment 301743View attachment 301744
Are apparently more distinct than these two:
Based on the reasoning of... Byleth holding their sword differently? In the middle of a fast-paced fight where you're in the air half of the time, about half of everybody won't be able to discern such minute little details, especially when they both use the color blue for their hair and use swords. If the point is the silhouettes, Marth and Byleth goes against that because if you just looked at the outlines, you wouldn't be able to tell who is who at a glance. Even then, the Summoner's silhouette would be distinct from Robin's by virtue of A: Having their hood on most of the time, and B: Using a gun instead of swords and tomes. Besides, a white cloak and a black cloak would be a lot easier to distinguish than just "Well my guy holds his sword behind his head."
You didn't address my point about the models and voice acting, either. That is also a fan rule, because you are definitively saying that the way Heroes functions disqualifies the Summoner from being added when there have been no official statements on the matter. It's not something that has precedent, like Assist Trophies or post-launch Spirit Events. Large cast gachas have not been represented in Smash, nor has Sakurai ever said that they are disqualifies, so you cannot say for certain that they are disqualified or how they would be implemented.
If you want to convince anybody, you'll need a better response than just a snarky one-liner.