Imo the battery life gain by windows on arm has been a minor achievement because laptops can be powered by usb c and usb c battery phone banks.
I don't see how USB-C-- plus related standards like USB-PD (that are probably in play for a laptop and that I would reasonably assume are part of this assertion, or possibly just USB BC for a phone, or at least older ones) are really relevant to this claim. It just tells you what chargers and cables your device can use. It doesn't mean it's efficient.
If you haven't been following, the latest USB-PD standards allow 240W (48V*5A) of charging, and ones allowing close to 100W have been around for some time now. Some laptops won't even charge from a 5V charger (which will max out around 15 W, possibly including some of the phone chargers and power banks you mention). In large part, this has just changed the connector and the way they receive power, not how much they actually use.