Not sure whether the tipping point as mentioned should even be used as an argument. That would be the same as assuming specific pixel ranges, not having touch input, or fast internet connection for "large" devices (as I point out in the article).

I think what matters is that it is possible to be responsive/adaptive based on such contextual factors. Responsive web design is basically enabling that for screen size, but there are many more factors in play.

A tipping point based on the feature-richness/robustness of a programmatic solution to go responsive is another story (i.e. it shouldn't prevent from going in that direction).