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It's not clear whether they're actually talking about domains or subdomains there, which is a worrying sign from a potential registrar.

Any domain that isn't one of the Top Level Domains is also a subdomain.

Isn't the actual top level domain an empty one after TLD? Looking like «.com.» with trailing dot

I mean sure, but if you started talking about google.com as a subdomain, real humans would correctly look at you funny.