As the post calls out, most certificate authorities will not issue certs against .arpa domains. I found this out the hard way when paying for CloudFlare's Advanced Certificates add-on for an .arpa domain I had successfully delegated and registered.

They return failures and exceptions when attempting to generate the certificate for the .arpa domain and then I spent months trying to get a refund (they are very slow to respond) and only caved once I threatened a chargeback for the service not working as advertised.