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.