Why can ssh-keygen export a public key in PEM PKCS8 format?
What you have here is indeed a structure called SubjectPublicKeyInfo. It's usually part of an X.509 certificate, but it is often also used separate from a certificate. It's, for instance, the default encoding for RSA public keys in Java and - if I'm not mistaken - OpenSSL.

You can view the complete structure here and compare it with the SubjectPublicKeyInfo structure defined here. Of course, you'll only see it up to the BIT STRING. The BIT STRING itself contains the structure defined as RSAPublicKey here. That the BIT STRING contains the RSAPublicKey structure is identified by the 1.2.840.113549.1.1.1 (RSAEncryption) object identifer (OID).

It's certainly not PKCS#8. That's indeed for private keys.



Note that what you are currently showing is the PEM encoding of said SubjectPublicKeyInfo. The PEM structure, sometimes also called "ASCII armor", makes it possible to send binary data over text interfaces, e.g. mail. It consists of the header, the footer and the base 64 encoding of the binary contents.