The signature is valid, but not legally binding - that doesn't make it invalid.
From citation 9 of the Wikipedia link
‘Signature’ of a treaty is an act by which a State provides a preliminary endorsement of the instrument. Signing does not create a binding legal obligation but does demonstrate the State’s intent to examine the treaty domestically and consider ratifying it. While signing does not commit a State to ratification, it does oblige the State to refrain from acts that would defeat or undermine the treaty’s objective and purpose.
This is standard practice, and nothing about the signature is invalid because there are additional steps to ratification. The bold specifically, again standard practice for modern treaties, places guardrails on US behaviour
internationally even through they aren't under purview of the treaty
internally. If the signature is invalid then you're claiming the US can flout the bolded obligation with impunity and other signatories wouldn't have standing to complain?