This is a standard DNS (name server) trick. Any TLD server can be programmed to have an "A" record for the tld. I have done this myself in the "Alternate Root" just for fun by tweaking a TLD server. That still allows A records for SLD.TLD combinations.
The reason this is typically frowned upon is that the DNS system is structured as a cached tree to take pressure off of the TLD servers (i.e., resolution starts at the TLD, with root servers pointing to TLD servers, TLD servers point to SLD servers etc.).
So in principal we could have
NamePros - Buy, Sell, Discuss Domain Names. , but there would be a scaling and performance problem if everybody tried to do that. Trees are efficient, flat lists not so much.