Well, it's a problem of mediocre information, and a lack of consumer understanding. Microsoft's site says you need at least a GTX 1050 for WMR Ultra. That's a 2 GB card. However, on the AMD side, you are required to at least have an RX 580, which is a 4 GB card. However, it is generally stated that the WMR Ultra requirements are the same as SteamVR, probably a partial result of aiming to support SteamVR through WMR in the future. SteamVR asks for a GTX 970 or an R9 290, both high-end cards of past generations, both 4 GB VRAM cards. It makes things confusing when you have "WMR Ultra is like SteamVR," but the requirements vary between the two and the sources you ask on WMR (Microsoft vs. this site). On top of that, you have the low-end WMR support with much lower system requirements. When Arizona Sunshine is asking for 4 GB of VRAM and Microsoft's site says a 2 GB VRAM card (GTX 1050) can do the job, things just get more confusing.
That said, it makes me not trust anything below the i7/GTX 1060 models of the Surface Book 2 for WMR Ultra, and because of the aforementioned CPU concerns, I will wouldn't recommend the thing to anyone interested in WMR--$2,500 as a starting point for the gamble you might need a better CPU? I'm not touching that. Don't think of 6 GB of VRAM as overkill either. A better card and more VRAM means the framerate is better. You are scraping the floor with the 4 GB cards recommended, meaning inconsistent performance, especially with higher detail settings, is going to be a real issue to consider (not that the game will be unplayable, but that you might see noticeable frame drops).
Don't touch the Surface Pro or Surface Laptop, though. In fact, I think the Laptop is an outright awful product NO ONE should touch in any use case (unless you're dying for an outdated chassis structure that fights against the very Pen support it builds in to the display...at a premium price that makes no sense). The Surface Pro might run some of the no-Ultra stuff, but that's going to be a lot of unimpressive, demo-like content and video streaming, not gaming. I'd step away from anything carrying a U-class process in a laptop, which means you're talking about laptops probably starting at $1,200 before you are comfortably clearing the performance bar for WMR. I might be wrong on that price, maybe it's doable at $900, but that's just a guess without looking into where Intel puts its HQ model CPUs.