I think they're talking about the Smash 4 Waluigi POTD.
If that is the case, that POTD was during the
base game's development. Meaning, it most likely did not have an effect on DLC development.
It should be mentioned that
ATs have been upgraded before. Little Mac and Dark Samus got upgraded to playable status in the next iteration of their respective games. So I personally don't think that image has anything to do with how fighter selection is really done considering an AT was upgraded in the same game.
I should also mention that in my opinion, I feel like the Smash team wouldn't limit themselves of their artistic vision. Game developers are artists, and most artists want as much creative freedom as possible. So because of that, I feel like the dev team would not impose limits onto themselves.
Heck, the team already disables ATs on certain stages for certain reasons, such as Alucard on Wii Fit Studio (because of mirrors LOL.) So there is precedent for them disabling ATs under certain circumstances.
I also think that Nintendo doesn't care about imposing limits as they are a corporate entity. If they want a certain fighter as DLC that's already an AT, they will request that fighter if they know it makes them money.
So... basically... in my opinion... I think there's more than enough evidence that the "powers that be" don't really care about that stuff. As long as it makes money to them, I think they're fine with it lol.