Unsafe by our standards, but not everyone is on SmashBoards, and not everyone uses our way of speculation. Sure, he might not seem obvious to us, but someone is bound to predict the most iconic JRPG villain of all time. I have a hard time seeing that Sephiroth is the single most unsafe character period when there's stuff like Sony, a bunch of Microsoft stuff that still isn't considered, a lot of indies, many obscure but influential old characters, etc. He was a shocker, but when SSBU is big enough to have over 20 million copies sold, someone's bound to guess him. You can't apply SmashBoards speculation standards to everyone.
And the source is... Anonymous. Nothing to back them up. Yeah, completely trustworthy. General rule of thumb, most textboard leaks are fake 99.9% of the time, and chances are this one's no different. Besides, there's a bunch of leaks that called things right back FP2, then fell flat. There was Doomguy, Monster Hunter, Hayabusa, and KOS-MOS; Each leak had called everyone up to that point, even Hero over Erdrick or Luminary, and then nothing. Just because a "leak" gets something right doesn't mean it's instantly credible.