I don't believe the problem you brought up is very important. Why do I care if there is a "constant, learnable latency", as long as each latency is reasonable? In the end I assume that the slowest response is the standard and any actions that happen faster are a pleasant surprise.
But more importantly this change doesn't preclude them from removing double-tap altogether in the future. I agree it is a half-solution now, but it's better than nothing if you are willing to grant that differing latency across pages is acceptable.
I do like the insight about stacking double-tap on top of single-tap, I'm just not sure how to apply it in the current world where single-tap means click/navigate and double-tap means zoom.