Deep pockets with fine corners are very tricky to machine, since it's hard to make an end mill that's all of long, narrow, and tough enough not to break. I'm sure it's possible, but it would be expensive, and that's before we even talk about the complex tool paths and consequent setup overhead that'd be involved. (And that's assuming there's nothing so deep it'd require EDM, in which case don't even bother asking, you won't be able to afford it.)

There's no mechanical reason why these parts need a fine surface finish anyway, since they're just display pieces. So it makes sense IMO to just do everything via Shapeways, since they already have support for a "white-label" manufacturing service that should make the whole process very easy.

(Hero Forge uses this, for example. It's not true white label since they do mention the partnership, but it may as well be; once you finish designing a model, you pay for the print through the Hero Forge site and a month later you get a box in the mail from Shapeways.)

That said, it looks like Github just gives you a .stl file to do what you like with; the prestige piece in steel looks to be just a one-off for the originator of the software they're using to do the "skylines" feature in the first place. In theory, I guess you could take that STL into Fusion 360 or something and turn it into CNC tool paths, and then you'd just need to find a CNC-equipped job shop that'll turn it into a paperweight for you. (Good luck! And don't expect it to be cheap...)

It's a shame the feature can't seem to see private repository contributions; most of the code I work on daily is hosted in private repos on a Github org, but those commits don't show up here, and that makes for a pretty sad result given that I barely touched any of my personal public repos last year. Nice soundtrack, though!