That rule is strictly enforced for employees. It is meant to avoid fake accounts. The work accounts (known as Workplace accounts) are created on a separate service and namespace. They are clearly considered distinct. You can toggle between them when authenticating, not unlike Google and Google for Business.

The particularity of Facebook is that the service emerged from internal use, so Facebook internal (a subset of the main Facebook) was (or is) tied to Facebook Workplace. The technical implementation was… hacky when I was there. I hope it’s cleaner now.

Every company that uses the service has a service that is clearly distinct from Facebook (with black rather than blue branding). I’ve worked for four client companies with their own, including two (subsidiary) at the same time, so I have my own personal account, the Facebook Workplace account is deactivated but my comments are still visible internally; three more deactivated accounts and my current one. I used to have to toggle between three accounts and that was rather seemless.