However, OneDrive doesn't seem to even know how to handle source files (it doesn't even attempt to honor the gitignore), so rebuilding a project in folders covered by OneDrive results in flurries of temporary files being uploaded/updated to the cloud, hogging the bandwidth, with a bonus of locking files, crashing builds, or even reverting changes silently.
One would think backup tools, even if primarily for normal users are still literally made by programmers, at least consider the use case of programmers instead of just ignoring them entirely.
file sync is backup against the things most people have to deal with (hardware breaking, theft, house fire, residential lightning strike)
I would say accidentally deleting a file is more common than a house fire, so NO, file-sync is not a backup nor is RAID (block-sync) or clustering (data/state-sync).
A backup is/should be a non-modifiable snapshot of data/files maybe even a state.
I give you an example:
You have a password-safe (as a file). Your computer blows up, but you are safe, because your password-safe is synced on Dropbox....but oh my god what was the password to Dropbox??? Your restore is gone because you forgot to "backup" your password-safe.
However you are not a Loser, being a Admin is a ~lost art and a really good paid one (again).... ;)