Though I think it's only strictly true, if the intervals you sample over are the same. Eg they both sample some messages every second, and the all start their second-long intervals on the same nanosecond (or close enough).

I find it easier to reason about reservoir sampling in an alternative formulation: the article talks about flipping a random (biased) coin for each arrival. Instead we can re-interpret reservoir sampling as assigning a random priority to each item, and then keeping the items with the top k priority.

It's fairly easy to see in this reformulation whether specific combinations of algorithms would compose: you only need to think about whether they would still select the top k items by priority.