Actually found smth interesting, at
page 274 it appears SEGA may have been supervising the book
"監修
株式会社セガ"
So yknow possibly more valid
Supervision is just something that always happen, it would be strange if there was no supervision
I mean, following up from the general discussion you all were having, it seems there are two extremes of "everything released is valid" and "only things that were done directly by SEGA/Sonic Team are valid", and that just isn't how development works.
SEGA will hire freelancers to do stuff for them, or license stuff that needs to follow licensing guidelines. Sometimes things end up not being caught in supervision and unreliable stuff gets released, but this isn't to mean that nothing done without direct involvement from SEGA can't be used, it only means that we need to take caution when using something.
Sometimes this feels like when Dragon Ball fans say the only DBS canon are Toriyama's notes and neither DBS manga or anime are canon due to that, this is just how work happens when it's not done by a single person.
Honestly, no different from fans that go with "Sonic Frontier's dialogue isn't canon because that was written by Ian, the only canon is Sonic Team's notes on the game".
This goes from the other side as well, Ian had a ton of work that has made its way in official material while he was working as a freelancer. Like, he wasn't a member of Sonic Team when he did the Sonic and the Secret Rings comic that was added with the game.
The truth is, franchise development is complicated stuff and there are a lot of branches and licensing stuff, we just need to have nuance while deciding what to use and try to find the most reasonable outcome, while accepting there are no absolutes.