I think he's wrong, but that may be what his humidity gauge is telling him. Many humidity gauges record the high and low marks over 24 hours. When he mists his enclosure, the humidity briefly spikes to 90% but quickly settles to 60%. His gauge indicates a high of 90 and a low of 60. It works for him, so that's what he shared with you. The gauge won't tell you for how long the humidity is at those levels, so it can be deceiving.
It's a stretch, I know. He's probably just a crackpot whose bp is weeks or months away from a respiratory infection. I just prefer to assume the best of people, at least until they've absolutely proven otherwise.