I think the logical explanation is that there was a mistake in the original article. A propeller is not the right choice for supersonic flight, I don't want to say impossible but it wouldn't be far off. They've gone from 3 jet engines to 4, and in both cases they would be turbofans.
The XF-88B Turboprop hit Mach 0.9 in level flight, but it was using an afterburner, which is rather cheating. Supposedly it could go supersonic in dives.
