That's correct. Either of them could collect telemetry on this. But, to what end? I expect that at some point one of the CDNs will want to advertise that this

That's correct. Either of them could collect telemetry on this. But, to what end? I expect that at some point one of the CDNs will want to advertise that this is a thing they offer. Once upon a time CDNs got a lot of blast for disliking the practice of claiming in TLS that you want some.ordinary.example as a way to conceal the reality that your HTTP transaction is for Host: censored.or.prohibited.example. One purpose of ECH is to enable them to deliver the same functionality but without all the technical problems. Now of course if they actually love censorship and so on, they'll do nothing, but it seems to me several big CDNs want to do this once they have a fix for the technical problems, which ECH gives them.