Interesting. It appears that there are at least two technologies being pursued here. The use of electric drives to separate the bypass air fan from the turbo-generator. And the switch to LNG (liquified natural gas). The latter is possible today to power gas turbine engines (the local power company uses CNG). The former has been proposed for (subsonic) aircraft, including some interesting VTOL designs with electric fans (propellors) and remotely located turbo generators (or even batteries).
Why we don't already see LNG jets is a question about the state of the technology. Electric propulsion is already a thing in some limited cases. But now add Mach 3 to the project and they are asking to have any one piece of technology stall the whole thing. Might as well have thrown A.I. in there while they were at it.