As Zubrin says, they’ve been deep underground on Mars for billions of years. They haven’t evolved to infect humans, or any animals living on Earth, and wouldn’t be a problem for us.
If he really said that, he's just displaying scientific ignorance -- one has nothing to do with the other.
Just make sure Mars isn’t put in the Ninth Circus Court of Appeals. They’d issue an order that humans can’t use the water, even if it’s OK.