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The x360 is not thinner than a 13 inch MacBook Pro (1.58cm vs 1.56cm, according to the manufacturers). And according to the following data sheet, the LDDR-3 memory on the x360 is soldered onto the board: https://www8.hp.com/h20195/V2/GetPDF.aspx/4aa7-6087eee

Soldering on ultrafast SSDs does make sense for a number of reasons beyond just saving space. The higher the bandwidth, the more difficult it is to get data transfer working reliably when part of the electrical connection is made via a connector cable. Apple have some of the fastest SSD speeds out there.

There are similar issues with RAM. There's lots of crappy RAM on the market. If people started putting third party RAM in modern MacBooks, you'd get lots of issues with e.g. reliable suspend/resume. Some good discussion here: https://www.quora.com/Is-there-a-valid-technical-reason-for-....

The laptop options that are hyped up as being better than Apple almost always turn out to have a mythical element, on further investigation. If you want removable RAM and SSDs you can get them (yay free market!), but there's a real downside.