Yes. It offers better scalability! E.g. at my university, our institute has its own IPv4 /24. That means a maximum of ~253 or so devices. We have exhausted that number and for every new client, an old one has to go. Now with a /64 this would be no problem at all.
To mitigate this the IT department is currently using VLAN-tagging for some of the devices. But it gets convoluted. And the more convoluted it gets, the higher is the chance for misconfiguration, errors, and security issues. Yet, they don't want to make the switch to IPv6 already.
He didn't say "all", he said "many". And that is factually right. Many ISPs use DSLite/CGNAT and don't hand out public IPv4 addresses to their customers anymore. Yes, some offer the option to change to a public IPv4, some charge extra money for this feature, and some don't offer it at all!
E.g. my ISP doesn't hand out public IPv4 and you can't order it, unless you change to a business contract. However, my ISP is doing some weird 1:1-NAT, so while I don't get assigned a public IPv4 to my router, I do get assigned a single IPv4 on the CGNAT router that also translates back to my home network.
Yes, it seems like it is behind the times. It doesn't matter much at the moment, as there are no real IPv6-only services of relevance (unless you need to connect to the private IPv6-only NAS of a friend or sth like that).
That just proves you haven't understood IPv6 yet. ULA addresses make it much easier, e.g. fd00::1 or fd00:2 (though this is bad practice, should be fdxy:zvwx:xyzw, xyvwz being random).
So same trust you put in any 3rd party DNS service. But I agree there's less contractual bindings to this service than an account somewhere that you even might pay some money for it.
Their business seems to lie elsewhere. This appears to be a minor service they offer free of charge to the IPv6-interested community, maybe to promote IPv6 usage.
No, I am not affiliated with them (though we follow each other on Twitter). My point is, I don't see any security implication involved with a wrong PTR record in relation to this service. If I set the PTR of my IP to this domain, but the domain itself resolves to some other IP. Or are you implying they can only request a cert if the PTR matches the domain? At least for LetsEncrypt this is not true, otherwise home owners with dynamic IPs wouldn't be able to request certificates.
Yes, you are missing something: S3 bucket resolves to Amazon's servers. <ipv6>.has-a.name resolves to the ip address specified in <ipv6>. You will have to install the certificate on the actual server that serves the webpage. For S3 bucket this is Amazon, so they can put their certificate. For your own IP, you need to install the certificate yourself, so they would have to hand you their private key as well, which is not allowed.
Same/similar working mechanism is built-in into Windows since Vista:
1234-5678--abcd.ipv6-literal.net
This doesn't even need functioning DNS and can work offline (add s<devnumber> for link-local addresses).