It would not surprise me if some of the manual copy came from the 9310 2-in-1 model, which uses a mainboard-mounted (soldered) X4 SSD -- totally different design than the 9310 clamshell model.  The 2-in-1 manual mentions an m.2 SSD, which it definitely does not have (the SSDs in the 2-in-1 models have been soldered for a few generations now). At any rate, if Windows is reading it as PCIe 3X4, that's what the interface supports, just as the manual states.    
Bought the XPS 9310 (2021 model i7-1185G7) with 512GB SSD, intending to upgrade the SSD with one I already had. Benchmarked the drive at 3500/3090MB's read/write which suggests running at PCIe Gen 3.0 x 4, but Win 10 device manager shows it is a Samsung PM9A1 which is a PCIe Gen 4.0 x 4, with speeds up to 6900/5000MBs. The i7-1185G7 supports PCIe Gen 4 if directly attached, or Gen 3 through the chipset. I asked support if I had a 4 x 4 SSD throttled by the chipset, or one running badly on direct attachment. Still waiting on an answer after some time, I opened it and found the following:-    [IMAGE] I can see PM9A1 and MZVL2512HCJQ, which relate to Samsung's PCIe Gen 4 x 4 SSD, but in the middle it clearly states "PCIe Gen3x4". Somethings not right, any idea's folks?
9310 SSD conundrum