Why would they publish this stuff.
Terrorists can read this stuff, get idea's and the next thing a know...
Terrorists have had their eyes on getting HPR motors since the at least the early 1990's, if not before....
Surprisingly enough the one known actual attempt to do so was by the IRA (Irish Republican Army), there was an arrest IIRC.
Anyway, an article mentioning buying a "$900 model rocket engine" or a fin can isn't something that would not already be on a terrorist idea list.
The really critical parts are the development of a Ramjet, and the guidance/control system. What I take from the article is that they were tasked with trying to develop a successful RAMJET engine for the intended size and range of the evenutal missile. So they were able ot focus a lot more of their hand-on efforts in designing and developing the ramjet. They did not need to develop a field-ready final missile design. So, they bought existing HPR engines and used either an existing fin can or morel likely had it custom-made by a supplier who could do so quickly and inexpensively. They got the ramjet to work, so now they can work on a final version which will be made in a more conventional fashion.
I have to say that more and more when it comes to terrorist possibly doing stuff with models, the "smarter plan" would be to use multicopters or "smart" R/C planes rather than Hobby Rockets. Easier and cheaper to do. Rockets would tend to be better for certain "airborne targeting", and some very specific kinds of ground targets where multicopters or smart planes would not be as suited for mission-unique reasons. Note I am not going to go into ANY details as to what those might be (and urge others not to speculate on any either), because yes indeed who really knows who might be reading this thread.
I just point out that when it comes to flying hobby models, possibly being used by terrorists, the allure of rockets as the primary means seems to have shifted now to far less likely due to the easier/cheaper (and potentially MUCH more effective) alternative of misused multicopters and R/C planes.
Having personally flown Quadcopters that can come back and land within 2 meters of where they took off from, really brought that home (so to speak....). And more recently using live FPV with a Quad and also with a R/C plane, to see straight ahead from onboard the model and use that view for piloting.
That does not mean we can be complacent about terrorists not wanting to try to use rockets. That will always exist. But it's not the only game in town for hobby models being misused by terrorists, and for the most part not the best game in town for "unwanted airborne delivery".