Well they're going to be shipping GOS on Motorola and I think some other devices, so basic things like the messaging app need to actually have some polish to them. I really like GrapheneOS overall as-is, but the messaging app felt like baby's first texting app. Not necessarily their fault since it's pretty much straight out of AOSP if I understand correctly. But any time I've installed GrapheneOS, the messaging app is one of two things I must replace every time (the other being the downright bad AOSP keyboard).

If they fix(ed) their messaging app and their keyboard, then they've really eliminated a lot of potential complaints if a less tech-savvy crowd ends up buying future Motorolas.

GrapheneOS has made a big push to hire a bunch of experienced, talented app devs, and they have been slowly chipping away at Messaging for a few months. They now have the resources to take on the AOSP apps and resume with the GrapheneOS apps. The rest of the AOSP apps are also planned to be overhauled.

They also plan to eventually support RCS in the Messaging app.

A bunch of the AOSP apps feel like you have to rewrite them for modern standards now and then you can basically have them in maintenance mode for years. The AOSP messaging app has also lasted for 15+ (?) years and has basically had no maintenance from Google for many years.

Modernizing these apps seems very high-impact. These are the very first things a new user sees and after the initial modernization, they should be relatively cheap to maintain.

I you look at the commits of the new Messages app it's mostly made by 2 people whose Github profiles show them as employees of GrapheneOS and who are mostly committing to different system apps. So I assume they have a separate team of devs whose job it is to make the system apps.

IIRC when they started first announced their replacement for the AOSP camera they said they hired a developer specifically for it.