They used to live in some others areas too. I once visited some places in the sierras close to Queretaro and while we were walking along the river a local guide told me he hasn't seen one in a decade but he used to see them regularly when he was a teenager.
Having said that there are surely a lot of factors that would make its survival impossible in wales given how hard it is for them to survive in their original ecosystems.
He may have been referring to the very closely related Ambystoma velasci
The historic range of the axolotl was indeed a bit wider than the current lakes beneath Mexico City, but not that much wider