I may have my own answer.
It looks like this test vehicle uses a single BE-3 engine, which only has about 110,000 lbs of thrust.
We have seen the SpaceX Grasshopper and F9R Dev1 vehicles take off from flat surfaces on a single engine as well, and the Merlin 1D engine produces 145,000 lbs of thrust (in 85% performance mode that apparently it has been running in for the first 15 or so F9 launches. SpaceX reports it plans to run it at 100% for GEO launches to provide margin from returning the first stage).
So we have seen that 145,000 lbs of thrust does not require an exhaust trench, nor a Niagara water system. Thus a 110,000 lb thrust launcher is probably fine.
On the other hand, we have seen the Saturn V (7.5 million lbs of thrust), the Space Shuttle (7 million lbs of thrust), and Falcon 9 (1.3 million lbs of thrust) do require both a trench and a Niagara system.
Would be interesting to find how high you can go without those systems before issues arise.