Wireguard isn't US-only in any way. Like, I literally do not understand at all. I can download a working, functional, largely stable Wireguard client on my phone or configure it on my Linux desktop without issue from outside the US. Like, US-based has no bearing on any of this. At all.
Where did you get an implication that Wireguard was US-only. The poster you are replying to is simply stating since Wireguard may still be a bit buggy, they are rolling this out in the US first to iron out any kinks before making it widely available.
But how does rolling it out US only first help with the fact that Wireguard is buggy.
They are already rolling it out slowly via a waiting list. Limiting that to US only doesn't really change how "widely available" it is in order to iron out the kinds. Seem more likely this is regulatory related.
Trying to do support in multiple languages and timezones can be tricky (and surely adds to the cost), I don't know for sure if that's the reason but it's a reasonable one.
Let’s take the fact that they don’t have the resources to launch to every user all at one for the reasons stated in comments near this one. Do you agree with that premise? If so, what do you propose as a more ideal limited rollout strategy? First come first served? That has its own share of problems with user burnout and people feeling annoyed after being left out as well.
I think you might be underestimating the breadth of i18n & l10n for a brand new beta, especially for an organization that is built on not making privacy or legal missteps. They only have 1 / 6 clients ready and zero customers yet, their attention has to be split all over the place, so limiting some variables in the meantime seems like a reasonable idea.
The VPN tech itself is not the only concern. Localization & payments & legal are also huge concerns beyond the US. This is also aimed at not necessarily tech-savvy folks, so that is also a concern.