Energy efficiency doesn't necessarily comes from being better on full load. Smartphonechips provide tons of power saving mechanisms, true but also are quite limited on how they operate.

I assume that generic code on a x86 desktop cpu should be more efficient by watt/work in comparison on a mobile chip. I'm also not sure how a smartphone is rated for 100% cpu load 24/7

That's most likely to be the opposite of what is needed here: do as little as possible and sleep as much as possible. Whereas the project actually needs is as much raw computing power as you can provide.

Yes, there are some methods to provide more computing bang for less energy buck, but this is a comparatively minor issue.

Not really actually, as a ARM cpu is not as powerful as a x86_64 one : the "compute power / electrical power" should not be really better I guess