this video was a fail imo.
an XServe 3,1 under the hood is effectively a Mac Pro 3,1 in server form. i own a 3,1 Mac Pro, and there's a LOT more that it can do than y'all showed.
for a start, 10.11 is the latest supported macOS version but it'll run all the way up to the latest release with patchers. with an nvidia card you'd want the High Sierra patcher, to install that, since that's the latest macOS that runs nvidia cards.
i actually had a 1030 working in mine on High Sierra, so theoretically it'll take any 10-series card.
furthermore you can very easily boot and install Linux on 3,1 Mac's so you could have done that. all the hardware in my 3,1 Mac Pro works on Linux so i don't see why an XServe would be a problem.