However, what happened was that he told the dep people "I will build a dep tool myself to understand more on the problem details", which implies "after I understand more, I will come back to you and discuss more", but not "after I build the tool myself, I will just integrate my version into the official tool". For this part, Russ was not executing his own words, or at least delivered the wrong impressions. Maybe this result should be already well expected based on Russ's personality or how the golang team did stuff in the past, but many programmers are not that politically awareing..
It is not the technical result that the "vgo" solution kills "dep" saddens dep people. It is rather the form of communication: "vgo" stabs "dep" in the back, by emerging suddenly out of Russ's pocket, and essentially claims immediate victory using the power of the Go core team. When the vgo posts are out, and the vgo proposal is posted (and it is not really bad technically), there is really no effective way for the rest of the community to reject it.
Russ could have kept his words by going back to the dep people and told them about what he learned from writing "vgo", and then move forward with integrating "vgo" if a consensus cannot be reached. Eventually, Russ might just be able to get the same technical results as today anyways (but maybe with larger communication overhead).
What the dep people essentially says here is that, if Russ really wants, they can also implement exactly "vgo" from "dep", but they never got the chance.