We use hydrogen as a secondary gas the the plasma guns which operate around 10,000 degrees and it doesn’t burn- the H2 disassociated to H+H in the ionized stream (releasing a lot of energy) and then recombine to H2
As stated the H-O-H bonds are very stong requiring a lot of energy to break them to produce H2 and O2, up until the OPs linked method was found. These strong bond do not allow water to be broken into its constituent gases in a fire. But when the two are combined in an engine they produce that high amount of energy and can propel spacecraft nicely when the reaction is properly controlled.
Hydrogen burns over a very wide range of oxygen concentrations. It is also the fastest burn of all the gasses- perfect as a propellant. It is ideal for HVOF spray for these same reasons. Methane (manufactured CH4) is a close second in the speed department.