"So if one of these subs runs up against a mine, the first they'll know about it is when it detonates, because they cruise around with their sonar switched off."

One of those subs might run against a mine and be sunk, true. However if any of those subs run around transmitting on active sonar they will all be sunk - equally true.

Passive sonars on both boats would most definitely have been on and manned, but these boats are very, very quiet. They say that surface weather conditions were rough which possibly created eough background noise to mask the submarines from each other.