Naoto Oshima retweeted this animatic today, I'd say this is definitely legit no doubt! Lots of neat stuff on his Twitter btw, like the following.
A couple days ago Oshima started tweeting never before seen animation cels of Sonic CD's opening cinematic.
And today the Sonic twitter account tweeted more CD animation cels.
Also,
Famitsu put their Sonic Mania interview with Iizuka up online. I'll jot down a couple things that may or may not be interesting.
-All of the bosses are new.
-along with Fang, Bean and Bark there's a poster of Espio apparently, anything Chaotix is interesting to me so I had to mention it.
-They're apparently hinting that the Eggrobo or Heavies may end up not being sub-ordinate to Eggman. Wondering what that's about.
-Iizuka mentioned preparations for the 30th anniversary.
-Also, Iizuka talked a little bit about the Sonic Movie and Tim Miller.
The Sonic Forces interview should be going up sometime today.
Others have already answered but there's a small analysis with the Hard Boiled Heavy here:
https://www.sonicstadium.org/2017/04/rumour-sonic-mania-to-include-animated-cut-scenes/