Gustafson is a noob. Both his Unum I and II proposals are completely impractical, making absolutely no consideration for how hardware would implement his ideas. But by writing only about the imagined merits he managed to convince a lot of people that his ideas were great.

Unum III/Posits can work, but mainly it is just a number compression format. Working with say 64 bit Posits pretty much requires implementing arithmetic equivalent to that of 80 bit IEEE floats, and then throwing away a larger or smaller portion of the mantissa depending on the exponent. And the extra accuracy around the sweet spot of 1 still comes at the cost of lost accuracy for large and small numbers, so some workloads will suffer.

Having watched that video, Gustafson seems to me much more vengeful and hostile than Kahan. There was one point--where an audience member was asking after the impact of variably-sized arithmetic types on implementing algorithms--where Gustafson basically says "you guys don't know how to code for modern computers anymore", which is when the moderator has to step in to keep things from escalating.

The answer to your question--why isn't Kahan working with Gustafson--is that, in Kahan's view, interval arithmetic (this is, AIUI, the main thrust of unums) isn't actually an effective solution to the "problem" of needing numerical analysis. While Kahan isn't the best at explaining this in detail, he does point out two valid issues: interval arithmetic can give excessively pessimistic ranges (because it doesn't account for correlated error), and it can give just plain incorrect answers when you have singularities in ranges.

I would note that, as Gustafson is no longer (as far as I know) pushing for the interval arithmetic approach, this is basically a concession that Kahan was right and Gustafson was wrong.