How are K-pop idols treated internationally? Are they Z-Listers?

Depends on the idols. But will they be treated similar to a Western A-Lister, then no. They are not.

Also mass popularity with market always does not go with A-listing.

For example, this was discussed sometime back in AKP - Indian tycoon got celebrities across the world to come perform for a family wedding - Rihanna, Bieber, Katy Perry and non-western artists like Rema, they got paid millions - but no Kpop at all.

Kpop does not even impinge on the consciousness of these customers.

But within entertainment circles, the fashion brands etc., - Kpop idols from BTS, BP, SKZ, TWICE have definitely made a mark.

So they definitely aren't Z-listers, but A-listing depends on the context. Some are in some contexts. But not as ubiquitously A-list in every context like Taylor Swift or Justin Bieber are.

They get huge huge crowds so it's impossible to just dismiss kpop idols at international events.

Le gala des pièces jaunes technicians acknowledged the power of kpop when they realised the decibel reading for the audience was louder then the sound system during the stray kids performance last year.