Seems like all Californians know this fact. That's why it's so shocking that they perished in the fire.

Adult trees also often survive fire; you can walk into any number of hollow, burned-out, still living trees.

Yes, but they are adapted to relatively frequent but lower intensity fires. As the article says, for the last century there was a policy of fire suppression, so the "undergrowth" (and from the perspective of a Sequoia, pretty much everything else is undergrowth) is much more dense than it usually was. So if a fire gets out of control, it may get more intense than the Sequoias are adapted to.

The article addresses that. This fire killed the trees.