>"Under" implies some ideal state of development
No, not necessarily "ideal". It simply implies there is an average or median level, with instances that are lower and instances that are higher.
>anything under that threshold is deficient
I think if you wanted a loaded term, you'd use something more like "development-deficient" and "development-abundant".
"Underdeveloped" doesn't mean "less than average development", it means "not and/or insufficiently industrialized", implying a problem to be solved.
[1] I saw it in person in Laos and also in this popular video https://fb.watch/nQ-Xvh9iLO/It seems appropriate to me to describe a slum with poor roads and power as underdeveloped, but weird to describe wilderness as underdeveloped. The former brings to mind philanthropic NGOs, the latter, villains from popular pro-environmentalism fiction.
It seems weird to me to refer to wilderness as "underdeveloped", but more because if it really is a wilderness then it should be undeveloped. If someone starts building things in it but few enough that the zone can be called "underdeveloped" then it's no longer the wilderness. In any case, what the original person said was that the country was underdeveloped. If there's not even a single road or rail between it and its neighbor, I'd say that's a fair assessment.
The “under” connotes that it is less than it ought to be.
Reminds me of the parable of the fisherman and banker: https://joneytalks.medium.com/the-parable-of-the-mexican-fis...