It's a get out clause in case some nasty comes along. Saves them from being sued. To understand the legal implications google Calill v Carbolic Smoke Ball Company, a case from 118 years ago but which still forms a basis of coontract law.
We only need some bacteria. I am currently reading a book called "How to be a Victorian" by Ruth Goodman. Infant mortality declined markedly as standards of hygiene improved. Scalding and using carbolic acid were the methods of choice but dangerous and terrible for the skin.Me I'll take modern products used judiciously.
If memory serves, you are not allowed to state, or imply, that your product is 'perfect' because that is not provable, so everyone stays on the side of caution, and never advises that any product will complete its intended task with no margin of error.
NOTHING !! That's the frightening bit. NOTHING can kill the super bacteria that lurk unseen everywhere and anywhere. It's like something from a horror sci fi film .Be afraid, be very afraid... they're coming to get you...
It's 99% or "all known". Fairly sure not both combined. Suspect it may be more to do with marketing sounding plausible than legal issues. Unlikely to be successful discovering a new surviving bug and suing.