Android 12 changed the meaning of allowBackup="false" to opting out of cloud backups. GrapheneOS encrypted backups use the device-to-device transfer mode which includes apps opted out of cloud backups. It's similar to the Google Play data transfer feature, not Google's backup system.
That still doesn't prevent other means that developers have to thwart backups. Chrome and vanadium has a custom backup agent that only dumps out settings, so browsing history and bookmarks aren't backed up at all. I believe firefox is similar unless they changed something recently. Same goes for other apps like signal. Browsers, messengers, and camera are the top 3 categories of apps I use on my phone, and the built-in backup gets 1/3 of them (camera actually works because it's just regular internal storage). That's not very good coverage.
Vanadium will add more data to the device-to-device backups but we haven't gotten to it yet.
For Signal, you can set up their own backups locally and they'll be included with backed up home directory data if that's enabled. Signal encrypts their database and encrypts the key used for it with the hardware keystore. A generic backup system can't back that up directly. The encrypted database is useless outside of the current app install since the hardware keystore key can't be exported.