That there is true. They did add Roy in Melee before Binding Blade was released after all.
Also, wouldn't characters like Inkling be considered "too soon" as well (even though they were never added in the game prior to their game's release). Don't get me wrong, they'd be pretty cool to have.
Sakurai works in mysterious ways. Then again, we all got character requests that we know may not even get in yet or not at all.
Here's my stance on Inklings.
There was a working demo of the game out before Smash 4 was finished, in the age that information goes across the world in INSTANTS.
The pre-release hype alone warranted the costumes. As the game has done so well that various retailers ran out of copies in japan and Nintendo had to do a public apology for underestimating demand, that is HUGE on a system that has habitually been poor in sales and demand.
The game will have been out 4 months before the ballot closes, and half a year to a full year before they'd feasibly come as ballot DLC.
I don't think it'd really be a case of too soon. If it was, would we have even gotten the costumes?