That's also right around where aerodynamic forces are juuuuuuust starting to transition to disassociation and ionization in high flow areas, as the electrons are ripped from molecules in the air. Probably not a coincidence, because a very ooooof zone will be the ramjet intake, since ramjets need to slow the air to subsonic for combustion. Lots of these high mach ramjets have amazing cooling devices on the intake.
These RDEs are especially neat because the exhaust mass is moving supersonically but they're very choosy about the mix in the combustion chamber, so and air-breathing RDE will be doing some sort of voodoo in that department. If they riddle it out they might get a single engine that functions throughout the airspeed regime.
My understanding is that mach ~5 is when ramjets hit their limit and you have to transition to scramjets if you want to go faster.