Is that necessarily true?

I'm 200% not a physicist, but it is possible that during transit, minor bumps / temperature changes / ionizing radiation / oscillating E-M fields could screw up the material in a way that matters?

I meant in the abstract sense. They seem to be able to make the stuff with albeit with a very poor yield.

Not necessarily true. Complex compounds can be susceptible to oxidisation and generally decay and degrade over time.

Apparently, they've offered samples to "serious experimental groups".

"Condensed Matter Theory Center (a Twitter account affiliated with University of Maryland) says they will send samples." - sanxiyn 2 days ago https://twitter.com/condensed_the/status/1684960318718406656

That would mean it's insanely difficult to reproduce.

Not necessarily. Just that the paper is rough and problems in synthesis aren’t well documented or explored.