Sequoia sempervirens seems to need both cool and moist to really do
well. In the best redwood habitat, it does get hot during the day, but
the nights are almost always cool. The moisture barrier is evident along
the redwood forest/chaparal border but there seems to be more than mists
involved in their survival. You will see redwoods in the S.F. Bay area,
where they get plenty of mist, but they are almost always very stressed
specimens. The cooler climate up north doesn't hurt the redwoods; they
survive beautifully in Seattle and farther north; the climate however
favors Douglas-fir as the dominant tree and the redwoods can't get the
upper hand.
As for conjecture - well, I'd love to hear other ideas too. Since we
have three genera in the Americas that are very widely distributed, there
must have been some central distribution point. We are here to share
ideas and learn things too, not necessarily to always be right.
Everyone's understanding of horticulture, evolution, botany, and geology
is not equal, and that is to be expected. No need to chide people for that.
Bob