I couldn't find this presentation, but from your brief description it seems like it is missing the point. Git has nothing to do with this - as a convenient tool it simplifies just one trivial aspect of it - namely rebasing the sources and storing revisions.
The vendor still has to maintain and test multiple versions, some of which are not under its control (the mainline versions). The procedure for replacing a mainline driver with an updated vendor version at a customer site is a huge PITA. Worse, some customers have the mainline one, some the vendor one, and it is non-trivial to find out which (most people are not kernel experts). You need different procedures for different cases and so on. This makes both support and development more expensive.
In short, for anything that is not a truly mass market product, it turns out it is actually in the vendor's and customers' detriment to have the driver in the mainline.
On a similar note, I have always found the notion that a driver in the mainline is somehow "better maintained" very very strange. Nobody actually tests all the drivers in the kernel before each release. All that is done is making sure they can compile. How can anybody be satisfied with that is beyond me.