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.
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.