I highly doubt it was just a coincidence. I see it more as Luigi was going to be revealed that day anyway, and the Direct happened to happen then.
What? Luigi was obviously revealed to promote Dream Team first and foremost. If there wasn't a Direct, why wouldn't he just be revealed as close to release as the other characters (Olimar and TL) have been?
However, similar to what Wariofan1 said, there's two times Sakurai reveals a character; On the release date of one of their games(not a past game, one that's releasing that day), or when an announcement for a key game is going to be released soon.
Luigi was probably planned to be revealed for Dream Team before the Direct was even planned, but when the Direct was formulated, and it was decided to be on the seventh, Luigi was pushed up to then, as it would have much larger reception as part of a Direct. The reveal's date was scheduled around the Direct, not the other way around. It's the same for Sonic. If there was no Direct, chances are the reveal would be closer to the actual release date, like all the others who didn't have a Direct, yet still had a game they were promoting.
I was surprised he didn't wait to reveal Sonic. But Sakurai and Nintendo working together for reveals wouldn't be surprise me.
If there was no Direct and they revealed him now, yeah, I'd be surprised too. But they wants to capitalize off the viewership of Directs, so really it's not that surprising.
And yeah, Nintendo and Sakurai are clearly working together for these reveals, I very much doubt this is the order they'd be in without Nintendo wanting to promote things.