Brabham is not revealing the source of the engine, but the 5.4L displacement makes me think it's a Ford V8 as seen in the first-gen (06-08) GT. This engine is compact and has a very stout and proven bottom end. Just remove the supercharger, add higher compression pistons, tune for track use only (no noise/fuel-econ/emissions) and 700 hp sounds reasonable.
There is another thread somewhere where the unusual firing order of the Ford v8 was discussed. Wonder if this firing order is on the Brabham engine and if it explains the sound. I think the conclusion from that thread was that the unusual firing order was for "acoustic aesthetics" rather than gas flow.