This thread feels like it's missing a lot of context.
A key is just a key - it proves nothing. It can be used for signing operations.
By analogy, if you lose your passport, you don't stop being you. The government that issued your passport doesn't say you stop being you. You've just lost a thing that helps with "proving" you are who you say you are - authentication.
The complexity in a lot of cryptography schemes is having to deal with lost/compromised keys: revocation.
