True, DO has no Pis. Why does it have to be a Pi, though? With the 1TB of bandwidth and the focus on IPv6, I'd think it would be geared towards networking, so that makes the machine irrelevant.
DO does, rent out decent machines that have IPv6 connections. At that point you can just turn off the IPv4 interface. Done.
To address your 2nd point: as far as I know, Digital Ocean's IPv6 doesn't provide the things you'd need to run an IPv6-only server (e.g. there is no NAT64 or DNS64 gateway, no v4-to-v6 proxy for letting the v4 Internet reach your server).
And sure, if you just want a little bit of computing power and literally don't care where from, you might as well get a VM from somewhere like Digital Ocean instead of a Pi from Mythic Beasts, but that is beside the point. This is an offer to rent a Pi on an IPv6-only network, which nobody else is doing.
I think you'd still be better off going with Digital Ocean or something comparable. Probably performs about the same for a lower price.