Low humidity is not likely a cause of RI bacteria do not generally same for spores thrive in low humidity conditions. That would leave only viral RI which thankfully is the rarest form. High humidity and poor air circulation promotes bacterial growth, and can absolutely lead to RI.
I guess my vet lied to me then.
Look at all of the RI's that people come to this forum asking for help on, to a T almost every single one of them are in tanks with low humidity issues and low temperature problems.