A total exaggeration. To get to California, the US would have double the land, true, but most of northern Mexico (which ITTL would be the western US) is uninhabitable, except for a few oasis towns like Lake Yuta City. Sure, there’s the oil in Northern Texas, but that’s insufficient to make the US a world police force like Britain was until recently.
Brittian may have lagged behind in the last few years but they still hold plenty of influence not to mention the coalition against Ruskies but the US could always grab Alaska if they have Acesss to the pacific plenty of oil there