They haven’t shown everything yet, it was just to see the main environment. They’re going to be showcasing gameplay throughout this month so they’re going to do it by showing it part by part: the layout of a island, then more structures, then enemies and combat, etc. If they jumped right into the action and such then they wouldn’t have enough to show for the whole month, and instead only have like 1 video.
And you can clearly see they haven’t shown everything by looking at some of the floating grind rails that don’t even look reachable in some cases or lead to nowhere, thus there must be structures in between them that they momentarily took out.
This is also only one setting out of about 4 that we’ve seen so far, and there’s supposed to be even more.
They put a lot of work into making the first open world Sonic game so they want to showcase every aspect of it separately to show all the details.