Addendum:

>> 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.

Wiki page up: https://wiki.amigaos.net/wiki/DMA_Engine

The misleading GART term is not mentioned therein.