[1] https://www.cnet.com/news/apple-macbook-air-vs-dell-xps-13-i...
Also note the 2019 Air they've got has a smaller battery again (49.9Whr).
The battery life gains in Macbook Air might come from the different series of processors used, rather than from the battery itself:
> The MacBook Air has Core i5 and i7 processor options, but they're part of Intel's "Y" series of CPUs, intended for thin laptops. They generate less heat and use less power, so you get longer battery life. The flip side is they're not as fast as a standard laptop with a "U" series processor.
Just looking at the ifixit.com page linked above, the XPS looks very well-designed and easy to work on too, with just 8 screws holding the back panel on, while still being just as thin as a Macbook. It seems like Apple stuff is simply designed specifically to be hard to repair.