Also, there is an assumption that the signature contains up to date information and/or does not change over time. The latter situation would else impact historical purpose. The signature has changed and does not refer to the position/information related to the moment of writing.
I agree with how both jwildeboer (Jan) and shykes (Solomon) approached this. Much appreciated in this case.
But yes, in a normal situation, this is irrelevant and the username signature is sufficient.