I don't think so, here is another example, what if the device counts how many times someone said "fuck", then sending {fuck:0} or {fuck:4,294,967,295} will result in the same size of data being transmitted. In fact imagining that the device is designed to do so, it could always send a large meaningless packet on querying for updates just so that when it actually needs to send data, it would look similar, same approximate number and lengths of packets and can be capped. I'm not saying it's the case now, just technically feasible and I believe hard to detect.

Also on "trusting" someone then answered in https://lemmy.world/comment/4594899 but I'd said it's also not "easy". At least one must trust their institutions able to vet on the person able to review such devices and that the device tested and the one used are actually identical.

Finally I'm not arguing for conspiracy theory or that Echo is spying on users, only that verification for privacy on closed system is not "easy" either through trust of 3rd parties or technical expertise for an "average" user, not somebody working in the domain.