Yes, Dell are doing their customers a disservice putting those SSDs in. Though, with only two PCI Express lanes, they wouldn’t be able to utilize the full speed better units anyway.
The 2018 XPS 13 seems to use more than two lanes as it gets the full 3.1GB/s reads according to this:
Again, the lack of ANY MENTION of the model XPS 13 they used in the testing leaves everything up to speculation and guessing.