From my understanding Dolby Vision may support HDR but it is NOT HDR. Also just because some minor players have it does not mean it should be supported by all devices. It is both unnecessary and redundant.
If Roku chooses to support it that is OK but if they do not that is OK too.
Dolby Vision is absolutely HDR. It's Dolby's proprietary HDR format. It differs from standard (non-proprietary) HDR10 in several ways. It can produce a much brighter image, it's 12-bit color vs HDR10's 10-bit color (resulting in billions of different color combos), it's "dynamic" (where each scene can be mastered for accurate shading) vs. HDR10's static "let the TV decide what to show" format, and it's backward-compatible to standard HDR-10. In other words, if you have a Dolby Vision device, you can also get standard HDR10 HDR.
As for "minor players", are you saying anything but Roku streamers are minor players? I'm not sure I get that.
I think what they are getting at is DV implementation of HDR isn't as common as just HDR. Are there any streaming services that offer DV?
Yes - Netflix, Amazon Prime, Vudu and iTunes.
Good article about HDR, and what sets Dolby Vision apart: Dolby Vision HDR