The signal:
Is it 15µV which is passed through an attenuator and then reduced again or is the 15µV present at the scope after the attenuator?
Speculating, but I think it could be 15uV RMS generator setting, and then followed by the attenuator, because 15uV RMS is easily discernible on MXO 4 with approx. similar span and realistic RBW. I had the attached screenshot on my PC, it is set to auto RBW, and it looks like it chose 40 kHz. Some number manipulation is needed on the screenshot because I happened to set the scale to dBuV and I wasn't using a 50 ohm setup for that particular test either. In the video, the RBW is 99 kHz on the MXO, and the HD3 has a bin size of 25 kHz or so.
From memory even 15uV RMS followed by 12 dB attenuator should still be discernible on the MXO, although likely with finer RBW on the MXO than currently set.
The spectrum 'capabilities' (I'm vaguely combining configurability/usability/features/performance) on the MXO 4 are exceptionally good (to the level that I often don't want to use a normal SA if I don't have to - and I have a pretty decent Keysight SA), so if the HD3 in any way approaches that MXO bar (and also has lower noise), then that's pretty impressive, since low noise on an ADC on it's own isn't sufficient.