It sounds like it is in addition to full-disk encryption, not instead of it.
Encrypting each field with a distinct key that an attacker cannot glean by simply exfiltrating all the data on disk and/or all the data in RAM protects against online attacks in a way that full-disk encryption cannot.
The real question is: does Salesforce do this properly?
It’s certainly possible that there’s a valid oversight here, but Salesforce has a rather talented security team, and the company truly lives by “Trust is our #1 value”^1
I can’t speak for the implementation, but my guess is that it’s been very thoroughly vetted by both internal security and external pen tests. They wouldn’t market a high profile security feature without that.
Encrypting each field with a distinct key that an attacker cannot glean by simply exfiltrating all the data on disk and/or all the data in RAM protects against online attacks in a way that full-disk encryption cannot.
The real question is: does Salesforce do this properly?