iirc, Pompeii was a pyroclastic flow [0], a fast-moving current of hot gas and volcanic matter with speeds between 100-700 km/hr and temperatures up to 1000°C. So, probably something like that, but a lot bigger, faster, and arriving faster from further away.

I was surprised how long the camera lasted getting buried. It'd be a not good end.

[0] https://en.wikipedia.org/wiki/Pyroclastic_flow

i just posted a sister comment to yours about the Mt St Helens explosion, with a picture from 1980, and then i noticed that they are calling (it's a non technical article) what rained down in the photograph onto the camera and photographer "pyroclastic flow" and it looks very similar to what happened here.

Yeah, they likely saw it racing down the mountain and then met their doom fairly immediately.