Why is it not the right place? Why do people even have to update underscore--just keep with the older version. They also have a mostly drop-in replacement in lodash (well, last I've seen, at least). So I think it's a perfectly good place, it's not a library of "shiny new features", rather a staple that most people probably don't need to update, and even has an alternative.
And you don't fight battles without a deal of incoveniencing. The mere fact of the publicity this is generating makes it a fair ground to fight that battle. Which is to say I can see his position, despite liking semver myself--though it could be distilled into a better standard and that could be a net positive long term on software development that far offsets the man-hours now being lost.