It's been around a while. 5-10 years. It's been inching it's way toward real hardware for all that time. As the article mentioned, the technical magic is liquifying the air between the engine intake and combustion chamber - in motion and at speed.
It will be a cute trick if it can be done without turning the engine into an ice cube.
It's been posted on TRF before. The British have made progress in static tests using a cryogenic liquid to demonstrate cooling the incoming air to make it dense enough to burn in the combustion chamber in the air-breathing engine mode. In this process of drastically cooling air, water or ice condensation is removed to avoid any harmful effects. In the full scale operation the air-breathing mode is used up to about Mach 5 than the engine switches over to pure rocket mode. Using LH2 to cool the air in air-breathing mode reduces the amount of LOX that needs to be carried aboard for the full-scale operation and makes a Single-Stage-to-Orbit vehicle possible.
It's still in the proof of concept stage. Many of the fundamental SABRE principles have been demonstrated as possible, and static motors have been fired, however a motor with the T/W requirements for a flight system has not yet been developed. Furthermore, the cross-section of an airframe that contains a large liquid hydrogen tank will be draggy as the density of LH2 is about 1/7 that of kerosene or a hydrocarbon fuel. The shuttle external kerosene tank would have been about 1/4 the cross-section the LH2 tank which had a very high drag coefficient.
The Sabre engine is by no means a new concept. It is described in the 1964 book, "Jet Propulsion for Aerospace Applications", 2nd ed., which references Astronautics, December, 1961 and Aviation Week, November 6, 1961. It has taken a long time to get to this point and it remains to be seen how well the concept works in practice. Originally, the concept was called LACE (liquid air cycle engine) and called for using liquid hydrogen to liquefy air, which is the method being done now. Tests have been done in England with the pre-cooler and more tests are scheduled to be performed in Colorado as shown in post #7. One earlier report talked about the importance of removing water condensation in the pre-cooler.