I can confirm that Ubuntu 18.04 amd64 with Gnome Shell runs on a VirtualBox VM with 2 GB RAM assigned. Would I want to run a Desktop this way? Surely not. Windows’ “basic requirements” (so not “what you really want” / recommendations) you linked are indeed 2 GB RAM for amd64.
So I’d say either the Ubuntu installer will need to introduce Gnome Flashback as a fallback for low spec systems, then you could keep saying 1 GB RAM as a minimum. As it stands now, I think you have to say 2 GB is a minimum.
Or you provide a recommendation. Personally, if I had to make a recommendation to someone planning on using the 18.04 (and 18.10, and probably also 19.04) default desktop, I’d tell them to have 8 GB RAM if they want to work unencumbered at basically all times. Since then, with most standard workflows, you won’t ever have to swap to disk, and can run multiple graphical applications, such as web browser with several tabs opened, email client with multiple accounts and archives, as well either a word processor, an image editor, audio editor, or maybe even a video editor at the same time.
I understand the need to state something similar to what the other OS state. As such, it probably makes sense to list minimal requirements, so 2 GB RAM, and to make it clear that these are the absolute minimum by wording it accordingly. (Sure, 1 GB may technically work, in a VM, but not really on a system with shared RAM / VRAM, not in the real world where you actually want to get anything done).