It started with this guy, who leaked several things about Devil May Cry 5. Now, I'm not very used to 4chan but apparently the guy didn't have a tricode or something, so there's no way to be sure it always the same leaker. But if you have the stomach to read all the discussion, you can easily see how he got SEVERAL features right, such as:
- Dante using some sort of "utility weapon" that's actually his motorcycle, like he did in the trailer
- "V" being a guy who looks younger than he should be, have tattoos like Nico's and using a staff to summon demons (only the summoning demon part is yet to be confirmed
- a training room similar to the one in DmC (around the middle of the page)
- how dynamic music will work (but this was already speculated/announced already)
- how save spots works, already confirmed for Nero

Now, mind you, guessing right some of that stuff is a thing, but by now it should be pretty clear that this guy really had legit info about the game.

Regarding Smash, about 1 minute after the first post, another post (supposedly the same guy) said some other stuff yet to be confirmed. Like Vergil being scrapped from Marvel vs Capcom Infinite and Devil May Cry 4 SE probably coming to the Switch. He also claimed that the Dante in smash rumor is indeed real, as he have seen the trailer. In a later post (again, supposedly the same guy) he described it as follow:
Some Fire Emblem characters are fighting a big dragon -> Dante appears to defend a female character -> gameplay of Dante, along Lady as an assist trophy, ending with his "ultimate move" (Final Smash I guess) -> end of the video where the girl ask his name, and he simply toss a rose.
The interesting thing about this description is that it follows several lines from other trailers, such as: a beloved Nintendo character getting killed in the first cinematic, followed by the titlecard ("Dante Rocks Out!"), then gameplay, and the end cinematic. If you notice, its the same pattern of other newcomers, but the post was well before the Simon/King K. Rool reveal.

So, to recap:
- the leaker is basically confirmed to have legit Devil May Cry 5 info
- supposedly, the same guy said he saw the Dante trailer for smash (and later described it in a credible way)
- there's no way to confirm he always the same guy posting, but the time from one post to another and the other legit info suggest that
- as usual, the devil is in the details: he said Fire Emblem character being attacked by a "giant dragon"; as he said he isn't a "Nintendo guy" its plausible he didn't recognize the FE characters, but that dragon is strongly implied to be the Rathalos (who, at that time, wasn't yet revealed to be a boss/AT), so how such a DMC/Capcom enthusiast cannot recognize him?

On top of that, there's Matt Walker. He answered in a tweet regarding Dante in Smash with a lol no, he is not, which is basically a deconfirm. Not because of what he said, but because that would break an eventual NDA. But in the entertainament industry, straight up lying about upcoming project is a common practice, like Jason Momoa lying about being hired to play aquaman during an interview (sorry I can't find the link).
Matt Walker and other Capcom producers already did that in a not too distant past, like for Tekken 7 Season 2 (announced 2 weeks after) or Devil May Cry 5 not being in the works (and yet look where we are...), so many people think that he's lying again to shut down the leak, for others is just confirmation bias, so take that as you wish.
There's also the Capcom Confidential Podcast where he behaved in similar way, but lying/confirmation bias work there too.

Finally, I want to point out that the original leaker may have just lied about Smash trailer, just to troll someone on the internet, but I've to admit he really did his homework. The (fake) trailer follows perfectly the theme of others trailer, included an Assist Trophy, included small details like Gilgamesh mask appearing on Dante's faces when Gilgamesh is used (that kind of things that Sakurai loves to put in), and specifically said that he use the DMC4 attire. And this tweet from Bayonetta/Dante creator came out 18 feb 2016, when the roster for SSBU was finalizated and the development began.