Thank you for asking!
Your question is the demonstration that "A Reverse DNS API" is a very subpar tagline.
As you correctly state, for "reverse DNS" usually we mean the PTR, which, not being a requirement for a working internet, is quite neglected. Few are sets, and those in large part are actually just an indication of which ISP or datacenter they belong.
What we try to do with Korbinsky is to provide information about hostnames actually pointing to a given IP address or just hosted there.
Just to give an example:
$ dig +short PTR 159.241.75.51.in-addr.arpa
Just to give an example: $ dig +short PTR 159.241.75.51.in-addr.arpa
ns3131293.ip-51-75-241.eu.
$ curl -H "apikey: l86WRM0NC2bBHMfEBQdkhXbDp2BoQw0O" http://ko.ckin.it/api/v1/ip/51.75.241.159
["petitbambou.com"]
I hope this clarify, and I hope, one day, to come up with a more explicative tagline.