It's not their number so much as their energy. The cross section for energetic neutrinos interacting with matter scales as energy squared, if I recall correctly. These neutrinos have cross sections too low to shield, but too high to ignore.
Related XKCD: "How close would you have to be to a supernova to get a lethal dose of neutrino radiation?"
Note that these neutrinos (from the collider) have much higher energy than those from a supernova.
Interestingly, that may actually make the collider-produced neutrinos less dangerous than those from a supernova. I'm not sure how it translates to other types of radiation, but at least for neutron radiation (e.g. cosmic rays) the amount of energy one would absorb peaks and then falls off at higher energies. Not harmless though, just slightly less dangerous.
Neutrino cross sections increase with energy, so these are much more dangerous than from a supernova. Note that when they do interact, much or all of the energy of the neutrino manifests in a form that can then interact further, so the more energetic neutrino also creates more secondary radiation when it does interact.
That's really interesting, thanks for the clarification
Yes, but I'm guessing you still can't blast someone on the opposite side of Earth with a stream of neutrinos going right through the core.