The NPR article also has no evidence for an earlier outbreak. They just report what the North's government stated, and add that the reader shouldn't believe them.

Sure they share a border with China, but China had COVID pretty well controlled for a significant portion of the pandemic. That combined with the DPRK's survival strategy of self-reliance make it seem plausible to me that they were clear of it until the vastly more contagious variant became dominant.

So far, there doesn't seem to be any evidence to the contrary.

It's always the same bullshit. If they are handling covid well "they're lying about their numbers". If they report high numbers it's "evidence they're incompetent."
What reason do I have to mistrust their numbers? They're not the ones having lied to me for decades.
And it's not like the US wasn't lying about its own numbers

Why would I trust the US to be honest about theirs? Why would I trust the US media in their claims about North Korea lying about its numbers?
The US had several whistleblowers like Rebekah Jones getting arrested/abused/harrased for their reporting on the state of the US obfuscating data.
The american media has been shown to lie time and again, especially when it comes to foreign matters - Most famously about Iraq. What reason do I have to trust it?
The United States has the largest prisoner population in the world and has a history of persecuting minorites and political dissidents like leaders of black lives matter. These dissidents are dissapeared at secret police blacksites where they are tortured. This prisoner population is used as slave labour, which is still legal.
Why would I trust the lies peddled by this authoritarian regime about a country whose population they relentlessly bombed until they'd murdered 20% of it.