And that discussion is laced with ignorance.
It's true that Google Sync, and Onedrive simply synchronize files, in and of themselves they aren't a backup.
But, the storage system they sync data into on the cloud side changes based on many things. If you're using M365 and syncing to any M365 tenant with a SharePoint entitlement, the individual user gets a SharePoint site they put their stuff in, then there's the Teams which create more SharePoint sites. Here's the thing about SharePoint... it VERSIONS and it has its own recycle bin, all of which are subject to configuration and policy.
If you have the last 15 version of a file in cloud storage, and it gets deleted... and it takes 30 days for that deleted item and all its versions to be purged... THAT IS A BACKUP! At least within the scope and scale of what your typical SMB thinks of a backup.
There is however one small detail further, never... NEVER think of the local copy of the data used to service the user's access needs as a "copy" of the data. If that data is changed, as soon as the sync agent finds the Internet that change will replicate online! It's not a copy of the data, it's not a 2nd copy, it's just a short cut to the living dataset in the cloud.
Google vs Microsoft here change nothing! Using FABs on a folder that's sync'd to the cloud is redundant work AND potentially destructive as when you connect the sync on a new install the files will appear again, and if you restore into the new folder you run the risk of wiping out the versions stored in the cloud in the process. So put the appropriate disclaimers in your documents, you cannot be responsible for configuration errors that previous to your work resulted in the sync agent; regardless of branding, not moving critical data. You however MUST NOT restore folders that were synced into the cloud back into their proper place if you back them up with FABS!
So feel free to make an offline archive, but the only person that can use it is your end user, one file at a time, should something be missing.