I can confirm that it only supports M.2 SATA.
Also, the 16 GB or 32 GB SanDisk SSDs that are usually included with them are really, really slow. I can get about 30-50 MB/s seq writes. It took me an an hour to complete a Windows 10 install on a T630 with 8 GB of ram and the 32 GB SSD (SanDisk U110 M.2 2242). I've got the same 32 GB in a T730 for OPNsense and it works well enough for that use case.
I've ordered some low cost used SK Micron M600 128GB M.2 2280 SATA SSDs as replacements for the SanDisk ones for my 9 T630. I expect them to be considerably better. Should get them in a couple of weeks.