Looking at how the paint settles and how the modeller can re-brush so many times without taking the paint off, I'd say either enamel or acrylic thinned with an extender (such as Future or equivalent).
Then the whole lot sprayed with Dullcote to flatten the paint and seal it all.
I saw this and the brushing it to flatten it made me wonder what mixture was being used. My vote is enamel. If you look at the white going on, it's not even that thin... The rebrushing is to lift off and distribute the excess. It's definitely something that self-levels much more than standard acrylics. If the technique relies on self-leveling, that's why everything's painted facing upwards. The rest is just a dark wash in the details and then a different tonal wash to give it some depth. I might give this technique a try sometime. The results look great.