The specs for the 8900 do in fact indicate that the desktop has an M.2 slot. Those specs are here. Go by the service tag and if both are shown as 8900s then believe that. I would surely not try an 8910 BIOS. There are plenty of forum posts about the correct BIOS update that went wrong. So why poke the bear and attempt a BIOS update that very possibly is the wrong one. I have advised before that unless the BIOS update fixes a specific problem you are having then leave it alone. Nothing good can come of an update that fails to install correctly.
I bought a couple of XPS-8900's several months apart many years ago.   The service tag when added to My Products shows BOTH as an XPS-8900. Cleaning the inside today on one unit I noticed the M.2 slot on the MB where I have a full length M.2 SSD.  The service manual for the XPS-8900 does not show that.  The service manual for the XPS-8910 does. I just recently (going strictly by service tag) installed the latest BIOS (2.9.1) without any difficulty.  Now I notice the 8910 has 1.1.12.   I'd like to think the BIOS update would KNOW what systems it is compatible with, but now I'm afraid to try the 8910 BIOS. The store receipt shows XPS-8900.   Hard to believe the service tag on each would be inaccurate.   Oddly, the 8900 BIOS (Feb 2021) is newer than the 8910 (Aug 2020)  
XPS 8900 or 8910, which do I really have?