Netflix and Amazon are founder members of the AOM that is developing and promoting AV1. If the smart dongle or TV supports AV1 they'll preferentially use that to get better quality at lower bandwidths.

Netflix and Amazon are both moving to AV1, so while that's true today, it won't be for long.

Smart TVs need to support VP9 for YouTube 4k. Netflix works with VP9 too, so HEVC doesn't seem necessary.