Came here to say the same thing. And note that you're "burning" 400MW of energy in the heat exchanger (transferring it to the helium) at some point you have to re-cool that helium back to liquid) if you outgas the helium you are a net loss because helium has a limited supply. Doing it with hydrogen doesn't get nearly the thermal mass. So it is a really really cool engine, but it remains to be seen how practical it can be.
The second thing, consider the experience from the SR-71. To stay light and still be able to withstand the frictional heating of the air at mach 5 requires some pretty exotic materials. And matching thermal properties for materials at mach 5 vs sub mach. Covering the whole thing in thermal tiles adds weight which comes out of the payload.
To me, adding on the possibility that it could be some sort of Mach 5 super airliner, suggests that the practicalities of it as an SSTO platform are perhaps not nearly so compelling as once thought. Fortunately they were not talking about the multi-gigawatt ground lasers to boil hydrogen. That idea, while nominally physically possible, is impractical to imagine the rest of the world being "ok" with.