I think the grandparent chose the wrong end of the stick with relating this to "famous" people, which, in turn, threw you off.

Sure, you can register an account in my name, but there are quite a number of people who will not be fooled: people who actually know me. People who know me in real life can tell whether an account is real or not, because they can tell whether I post about things I do, whether I post pictures that are...well, me.

In that case, they can be reasonably sure that the account in question is, in fact, my account. If I attach my GPG key to this account, they can thus also reasonably assume that the GPG key belongs to the account that belongs to me. This essentially gets you the online equivalent of a key-sharing party.