I didn't read the article, but it's likely they are faster than light is when traveling through that water. light can have different speeds through different media.

for charged particles (so not for neutrinos) this leads the Cherenkov radiation, often observed in the cores of nuclear reactor: https://en.wikipedia.org/wiki/Cherenkov_radiation

as to why in this case: it can be somewhat intuitive to think that photons would be forced to take a somewhat longer path when traveling through a medium they interact with a lot (water, anything with a charge) then something which is almost going at the speed of light and famously doesn't interact with almost anything!

[I was going to write an explanation, but these sentences of Wikipedia article ar better.] From https://en.wikipedia.org/wiki/Super-Kamiokande

> A neutrino interaction with the electrons or nuclei of water can produce a charged particle that moves faster than the speed of light in water, which is slower than the speed of light in vacuum. This creates a cone of light known as Cherenkov radiation, which is the optical equivalent to a sonic boom. The Cherenkov light is projected as a ring on the wall of the detector and recorded by the PMTs.