Registration of ip6.arpa DNS zones

Not at all.

In order for you to publicly provide DNS data, you have to OWN the zone, officially. So the same way you have to buy a domain from a registrar for forward lookups, which then means that all the way from the root DNS servers to your zone every participant at each level knows where the next authoritative server is. The root servers know which servers are repsonsible for com, those know who's responsible for mycompany.com etc. etc. because you have registered that domain and all the necessary information has been provided to the superior organizations.

It works roughly the same way for reverse DNS. If somebody has registered a block of IP addresses, somebody else cannot just go and serve PTR records for IP addresses in that range. Or more properly, you can serve them, but clients won't ask you for them, because all the way from the root servers, the clients won't be directed to your DNS server but in this case Amazon's because it's their registered IP range.

So, either you have your own IPv6 addresses and configure everything to use those (and then have your own public DNS server with the reverse zone) or maybe you can upvote over here:

https://repost.aws/questions/QUgROEsvrGTPKPAXFVCf8xwA/please-allow-reverse-dns-for-ec-2-i-pv-6