The video is very old and most Android devices didn't use disk encryption by default, so a physical extraction (image of the entire flash storage) could allow recovering deleted files from carving unallocated space. Nowadays Android uses a "file-based encryption" (FBE) where all data is encrypted with separate derived keys for each file, directory and symbolic link. Deleting the file loses the keys and recovery is impossible.

If you can recover data that is deleted from an app, it means the app is caching it when it shouldn't be and it's a flaw they would need to fix. I don't recall this being an issue with Signal but if you can extract the app data before the message database is rebuilt for deleted messages then you'd be in luck. You could kill an app and prevent it cleaning up it's DB. This is something you can apply to every messenger though.

Getting this data requires as much as a full filesystem extraction (FFS) to extract the application /data directory where the message databases are. Cellebrite has no extraction support for GrapheneOS according to themselves. No specification on what the most they can extract from an unlocked device is, but assume that all forensic tools get this data anyway.

Molly lets you encrypt the message database with a passphrase, so it wouldn't be accessible regardless of if there was a FFS extraction and a flaw in Signal keeping the messages.