Separability vs. Resolution - Synonymous terms?
Most technical terms, such as these two, do not get their definitions purely from their etymology, but rather from the context of application, by experience and by tradition of acceptance. 

And for this case, your understanding of separability as the allowence (or ability) of someone to distinguish between two closest time of arrivals seems a synonym to the more classical term resolution in your context. Nevertheless, they will (and can) take their absolute precise definitions only when they are given mathematical expressions to be quantified...

Note that, in multi-dimensional signal processing, the term separability assumes a completely different meaning too, and which is not related to the discussion here...