The question is energy-efficiency.

There are a number of possible ways to make salt-water burn, and, as you can see from the video, it takes continuous application of radio frequency to keep the water alight.

Perhaps he's stumbled on the proper microwave frequency to excite the sodium ions in sea water... the release of hydrogen and sodium from sea water provides fuel for the flame in the test tube.

The big question is... how much radio frequency is used? And how powerful is the transmitter? If the machine uses over a thousand watts of power to light just one test-tube flame, you're still using more energy than you put in... so current practical applications are really limited.