My layman ass is imagining that if the shells are of the same calibre, it should work out.

See, my layman ass thought much the same, but then AFU began reporting issues with using nato shells with nato guns, despite calibre being standardised.

DPRK is sending old soviet shells to the Russians who are then using old soviet surplus artillery to fire them

This I would be more inclined to believe. However this implies:

1. That DPRK is confident enough in their ability to either replenish their stocks or obtain new weapons quick enough so as it doesn't become a problem

2. That Russian military has either ran out of their own soviet made shells or can't replenish them quick enough (the latter I could believe tbh, given how much damage has been and is still being done to our industrial capacity)

3. Those shells and weapons, despite being 30+ years old, are still not only functional, but quite effective

There are just so many factors here that if it somehow turns out to be true, I'll have no choice but to eat my hat. Because boi!