It's not a great as the article seems to think, it's around 500 secs from hydrolox according to Scott Manley. A few pictures I've seen make it look like an aerospike which makes sense with an annular combustion chamber, so may well be good for a first stage or an SSTO. Another concept I've seen recently is a sort of nuclear thermal/ion hybrid with an ISP of a few thousand whilst generating serious thrust, this makes more sense for deep space.