There can be external candidates with that qualification. In general the standard is “Bona Fide occupational qualification” which means that you have a legitimate reason for the requirement. For a college hire this would likely not suffice ... you can teach a college hire what they need to know. For the engineering director running the project it very well might be.

Incidentally, a bunch of people outside Microsoft had access to these tools (e.g. academics).

The job posting lists it as something preferred but not a hard requirement.

Don’t forget former employees.