The problem isn't that afraid.org can't serve IPv6 glue. It can and does.
It can't serve TLD glue records, which is what the Sage test looks for. I haven't found a way that it could serve more pedestrian non-TLD glue records, either, but that's neither here nor there for the sage test.
Put a more technical way: Say you have a .com domain at afraid.org - yorick.mooo.com, why not. And your nameserver is ns1.yorick.mooo.com. What the Sage test looks for is an answer to "dig ns1.yorick.mooo.com AAAA @a.gtld-servers.net", essentially - a AAAA record for your nameserver held at the registry for the TLD. That is TLD glue, and afraid.org certainly can't provide it. Not with AAAA nor with A.
A more "pedestrian" glue might be to set an NS record at afraid.org for yorick.mooo.com pointing to ns1.yorick.mooo.com, and then have an A (or AAAA) entry at the mooo.com level for ns1.yorick.mooo.com, so it can be found. I don't see a way to do that in afraid.org, either, but maybe I'm missing something in the web interface. Even if that is possible - and if so, I'd love to learn how it's done in their interface - that's still not TLD glue, and won't get you through Sage.
Whether afraid.org's servers are reachable over AAAA is rather beside the point. The glue has to be at TLD level for this test; the afraid.org name servers would never get involved in the query, anyway, if TLD glue could be provided.
To be fair, this is not afraid.org's fault in the least. I'm not even sure a subdomain service like this _can_ get TLD glue entered for all those subdomains. I can see the operators of the TLD registry objecting to such an idea.
The discussion really is only about "what is TLD glue", and "how do you get it", and possibly "and then how can you complete Sage on the afraid.org subdomain you started out with, once you have TLD glue" - not about "whose fault is this" (nobody's, truly) or "rabble rabble afraid.org should" (they should be given lots of beer for providing an awesome and free service).