It isn't a validated build. It works the same, but it isn't sufficient if you actually need a validated FIPS module.

> It's just the library, nothing special about the OS.

If you need validation, then the kernel also need to be validated.

Now, it isn't impossible to get a FIPS validated system working with Debian, but you will probably need to contract with another commercial provider like WolfSSL to get validated components. Or compile things yourself. But that is more complicated and involved than just paying for an OS that is validated off the shelf from Ubuntu, Red Hat, or Suse.