The press release is much clearer.

http://www.osa.org/en-us/about_osa/newsroom/news_releases/20...

I haven't read the paper, but it appears they're proposing to use the ablative lasers only to shape the flow of supersonic gas through a thruster nozzle.

That's the question I have. I presume they mean the speed of sound at sea level, which would most likely be many times greater than the speed of sound in space.