It's still a custom ROM and it's not uncommon for those to be reskinned in interesting or weird ways. Plus, their website already uses the outline of a phone with their logo inside of it, why not make that a screenshot of their ROM with their logo as a background?
Furthermore, I'm interested in what their solution for email, calendars, and all the other Google services look like. If it's AOSP, that's a good reason not to use this ROM. I suspect it's not actually using AOSP's apps, though.
> Furthermore, I'm interested in what their solution for email, calendars, and all the other Google services look like.
There are no built-in apps for that. The only custom apps GOS provides are Auditor (https://attestation.app/), Camera, Calculator¹, Contacts¹, Files¹, Gallery¹, Messaging¹, a hardened PDF viewer, and Vanadium (a hardened version of Chromium). Everything else is up to you.
Furthermore, I'm interested in what their solution for email, calendars, and all the other Google services look like. If it's AOSP, that's a good reason not to use this ROM. I suspect it's not actually using AOSP's apps, though.