I mean... yeah, you obviously have to trust
someone to vouch for the authenticity of an identity. In the case of Github, that's the platform owner. In the case of a digital signature, that's the root certificate authority.
With that being said, your example feels pretty far off the mark. You might be able to phish using a similar looking identity, but that's completely unrelated to the trustworthiness of the platform. It's not as though you'll manage to somehow phish Github into showing someone else's trustworthy work history on a spoofed identity.