I believe you're right about the Quake engine, looks like Q3 to me. I wonder why there is no mention of the engine at all, and much more, the license.
Edit:
Interesting, the lead programmer game designer wrote at ModDB:
"It is a custom engine developed by us. Maps are done ingame with the dirtgun and some special commands to paint different materials. Thats mostly all to it, we will write a more detailed howto so anyone can easily make maps."