Understanding this particular criticism of RCTs by Deaton and Cartwright
Without having read the full piece I'd say you are mostly spot on, but for a subtle difference. The objection runs a bit deeper than your rephrasing of it, where you refer to single estimate. The thing is that there are way more than a single estimate this applies to, and therefore you are more likely to be off.

The point is that for RCT to establish causality both groups have to be equal on all other characteristics except for the treatment. By doing assignment randomly one can expect that in a large number of potential samples for all these aspects the independent samples' averages will be close to the population means for these characteristics.

The problem is, as you rightly point out, one may be unlucky and have samples where the sample averages are not actually close to the population means. The additional problem is that sample averages have to be close to the population means for all variables that matter, but 1) you don't always know which variables matter and 2) given the large number of variables that do matter you'll always have one or two where you are "unlucky" in terms of the sample averages not being close to the population mean.