This kind of inference can be interesting, because occasionally people try to convert a private key from one type to another; for example, there are scripts to convert a private key between PEM, OpenSSH, and GPG formats so that you can re-use it for different applications. (One application for this is to use GPG signatures on public keys to authenticate users and/or servers in SSH.)

So this research might allow someone who didn't already know to recognize more about where a key came from.

Verification that TLS keys are mostly generated by OpenSSL and PGP keys by PGP/GPG are sanity check that method actually works.

If it will turn out, that particular library has vulnerability, one can quickly search for other vulnerable keys from large datasets like IPv4-wide TLS scans.