The article is a bit unclear, but we have both a very wide range of X-ray vs charged particle studies, and increasingly of conventional vs FLASH studies with a range of modalities (e.g. the seminal FLASH paper was FLASH electrons vs conventional electrons). FLASH photon vs conventional photons are also increasingly being generated, although they've been more of a pain to generate.

So it's clear there is a temporal FLASH effect, which is not purely a question of radiation type.

That's not to say it's necessarily exactly the same effect - we still don't have a perfect quantitative understanding of the effects of different radiation types even at normal dose rates, let alone when FLASH differences are added into the mix.