It’s less that this allows new unprecedented weapons, and more that this allows one to validate material properties in similar conditions to a nuclear bomb detonation without actually testing a full scale device. This lets one verify and refine the otherwise “magic number” constants in computer simulation code that were empirically derived 50-80 years ago.
Of course, if you’re a nascent nuclear power along the lines of NK, you just do full scale tests, treaties be damned.
North Korea hasn't signed any treaties abolishing nuclear weapons tests. In point of fact, the USA itself hasn't ratified[0] any treaties prohibiting the type of underground tests the DPRK has conducted. The US chooses not to do those (and has maintained a voluntary moratorium since 1992), but is under no obligations.
Notwithstanding the US's deeply hypocritical stance on treaties it hasn't ratified but others must follow, North Korea did in 1985 sign the NPT which forbids them even building nuclear weapons, as well as the 1992 "South-North Joint Declaration on the Denuclearization of the Korean Peninsula" where they explicitly pledge not to test nuclear weapons (which they indeed didn't do until 2006).
An agreement was made to suspend the nuclear program in 1994, the US then refused to certify it after 9/11 due to NK's transfers of technology to Iran.
Whatever anyone thinks about that, it's got nothing to do with the 1994 agreement (which is easy to read, it's only a few pages).
NK subsequently withdrew from the NPT, and got a nuclear weapon in 2006.
> South-North Joint Declaration on the Denuclearization of the Korean Peninsula" [...]
I believe that NK's stance on the matter is that SK is hosting military bases for a foreign nuclear armed power (the US).
So it's implicitly threatened by a US nuclear attacks, and that threat became explicit under Trump.
You know, the best part is that US is doing this same thing with the incoming president shitting on the work on the previous president for the sake of domestic politics, and not getting a new deal with the belligerent state!
So instead of Clintons deal Bush tore it up, didn't get anything, and now North Korea has nuclear weapons. And instead of Obamas deal Trump tore it up and got nothing. Now Biden also wants a new deal, so we know how this version of story likely ends (especially with Bibi bombing out of control, the pro-nuke factions in Iran have great fuel for winning the go/no-go argument on building a weapon.)
Of course, if you’re a nascent nuclear power along the lines of NK, you just do full scale tests, treaties be damned.