iPhone X size classes

If you're to follow Apple's documentation in theory it should be h = compact, w = regular vs h = compact & w = compact but given the device is smaller than the Plus models it falls into compact, compact.

https://developer.apple.com/documentation/uikit/uitraitcollection

Because, even though the screen size is bigger that the iPhone 8, the Safe Area of the iPhone X is really more like the screen size of the iPhone 8. You can check out the Safe Area for iPhone X in XCode 9. So the key thing here is the new Safe Area designation.

In summary, iPhone X Safe Area == iPhone 8 Screen Size, and hence they both have the same size classes.

Hmm you might want to read this in more detail, Apple makes the assumption on if the physical size is smaller than any Plus model device when in landscape it will always be compact, compact - this is not based on the pixel size or screen resolution of the device.

Default Size Classes for Different Devices

https://developer.apple.com/library/content/featuredarticles/ViewControllerPGforiPhoneOS/TheAdaptiveModel.html#//apple_ref/doc/uid/TP40007457-CH19-SW1

The link you provided for (Default Size Classes for Different Devices) was last updated 2015-09-16 🙂 So this link cannot possibly be relevant here 🙂

In any case I looked at this old link and did not find anything to backup your claim that "Apple makes the assumption on if the physical size is smaller than any Plus model device when in landscape it will always be compact, compact".

The closest thing in the old link that hints at class sizes is "Devices not listed in the table have the same size classes as the device with the same screen dimensions." So based on this quote, and since iPhone X is unique as it does not share its screen dimensions with any other Apple product, the link you provided is again irrelavant based on content, in addition to it being outdated as stated above.