This is indeed a good question… OK, I could imagine hydraulically actuated disk brake calipers, but in that case, the hydraulic "system" would be closed, in the brake mechanism, and (very simplified) a pneumatically controlled hydraulic cylinder.

FWIW, I was tempted to put the "hydraulic brake" down as an invention by a journalist, but the report apparently comes from a source where at least some technical correctness can be expected. (this opposed to a recent report in a regional rag in Germany, about a locomotive with a transformer fire, where they said that hydraulic oil was burning… well, yes, if you put hydraulic oil in a transformer, you may have to expect fires…)