They could have made their own, but it's really not that uncommon for music to slowly increase in tempo. Some styles of music use it more than others - for instance, ceremonial drumming might start out extremely slow and build to a frantic climax. But it's also fairly normal for musicians who are playing live, rather than to a click, to speed up when the music gets more intense.

I'm all for ceremonial drumming but there's gotta be a specific pace that fits their definition of "progressively" and i feel like that detail is lacking. Maybe there's more in the paper proper but the appendix is quite vague.