If California & New Mexico belong to Mexico who signed the Treaty of Guadalupe Hidalgo?

Nobody takes such claims seriously so your question is irrelevant.

Let us be very clear … prior to about 1900, the boundraries of all nations were determined by force of arms. The winner drew whatever boundraries he liked and the losers accepted whatever was left to them.

This was also so throughout Latin America. The various uprisings against the Spanish overlords centered on the most populous regions, usually separately or in sequence, and resulted in separate nations whose boundraries were drawn by agreement between the winning revolutionaries and the losing Spanish.

I rather suspect the boundrary between Mexico and Guatemala was similarly drawn and reflects the military power of the central Mexican plateau as opposed to that of the mostly Mayan descent Indians of Guatemala.

So, when we hear noise from Mexicans about Alta California y Santa Fe de Nuevo Mexico, I think we should notice that Mexico has no intention of returning the Yucatan Penisula to Mayan [Guatemalian] overlordship (or freeing the Mayan majority there from Mexico City) …

thus — the Mexicans in question are speaking with forked tongue and simply are political opportunists.

**
It is interesting that they are being heard by liberal groups in los Estados Unidos de NotreAmerica with something like approval. I suggest that this is mostly guilt feelings by people who are economically successful without ever having earned their success.

They could, of course, alleviate that guilt more easily and quickly by giving away their inherited wealth and going to work like the upward striving classes — but that, of course, is beneath them.

The people who were being slaughtered in the name of land, manifest destiny/white is right