[paulgb]

Because different tlds are operated somewhat differently and afaict this seems to be a secondary service that isn’t fundamental to routing or other required operations of a registrar.

In this case it seems to work with “participating tlds”, whichever those are.

[phgn]

All DNS zones on CZDS have the same format, and AFAIK all nTLDs are required to use it.

But you're right for ccTLDs -- everyone uses their own format and most don't even allow you to get the zone file. There are some companies though that provide similar lists from crawling data (e.g. https://zonefiles.io/).