BT.2020 color primaries are designed to cover much wider range of CIE chromaticity than BT.709, and also it's used for both SDR and HDR contents. So, the incorrect assumption (i.e., BT.709 as a BT.2020) is risky and resulting image color tends to be visually very wrong. Unless there's obvious clue, don't consider color space of high resolution video stream as BT.2020