These points are kinda putting Iizuka in a positive light, I've lately been of the impression that Iizuka was a bad influence on some games in the franchise this past decade :X
Try two decades, Iizuka's more-or-less had the reigns since directing the first Sonic Adventure. He's been the head of the Sonic Team USA offshoot (international release of Adventure 1, Adventure 2, Heroes, and Shadow the Hedgehog) and then became the head of Sonic Team in 2008 (starting with Sonic 4 and Sonic Colors). The only Sonic Team-developed games he had no real involvement in were Sonic 2006, Unleashed, and the Storybook titles. Same goes for the Advance and Rush eras of Dimps handheld games.
That said, since he's the studio director, he says everything official related to Sonic goes through him, and the Mania dev team members are also working under contract. It's not like they can or allowed to say anything against the leader of studio in charge of the series and gave them the greenlight to make the game in the first place.
Iizuka just got lucky with Mania. The guy is poison.
Iizuka's involvement in Mania is largely supervisory. He's said in an interview that the team comes up with the game ideas and level design first, and then submits it to him for recommendations/approval. Outside of the name and the final game concept (having remixed old levels as well as new levels), he's otherwise left Taxman/Stealth/PWG to their own devices.
Note how Sonic Team hasn't been actually named as a (co-)developer for Mania (in contrast to the Dimps-developed Sonic games), and how the game has been officially billed as a co-production between the development team and Sega (of America).