Update:

> What I find surprising is that GART support would fall in the domain of ExecSG.
> I would have located this within the graphics framework.

Mystery resolved. kas1e, who managed to watch all 2 hours, explains that what the ExecSG team lead calls GART is not real GART but the usual kernel DMA that's been implemented on the X1000 for ages but is still missing on the X5000.

https://amigaworld.net/modules/newbb/viewtopic.php?topic_id=43473&forum=14&start=60#828212

Reminds me of those "eggheads" and their "textbook implementation of SMP" ;-)