It does this by not only inhaling air, but also water, as oxidising agents.

Since the article is a little bare on scientific details about what is going on, I found some extra articles

The ramjet injects water vapour into its combustion chamber where it reacts with boron and other metallic fuels like aluminium

Many metals have an exothermic reaction with water at high enough temperatures, this produces hydrogen gas and metal oxides.

In this case of this ramjet, I am guessing they used a variety of fuels to “step by step” raise the temperature of the chamber. This is because aparantly, boron reacts with water at 700-800 C while aluminium reacts at room temperature.