Seriously, a petition wrote itself? Do Khanna, Sanders, unions and the usual suspects in the party vigorously support this insanity?

It didn't write itself. It (mostly) came from the SEIU, whose members would benefit the most from a big pot of new revenue that could only be spent on health care.

I am sure that there are some individual Democrats that support this measure, particularly from the economic left wing of the party. But that is very different from suggesting that Democratic politicians or the Democratic party as a whole supports this measure. Generally, they do not - again, which is why the California Legislature never tried to adopt something like this (even though that's much easier than a referendum), and why Newsome and other Democratic leaders do not "vigorously support" this measure.

There are a lot of institutional reasons why the Democrats do not, as a party, end up burying the needle on trying to "eat the rich" - notwithstanding the fact that there is a non-trivial portion of the party that very much would like to. It's one thing to note that there exists some wing of the party that wants to go hard on taxing the rich, but it's an entirely different thing to suggest that it's realistic that the party would actually adopt measures that go hard on taxing the rich. There's a very good reason why the party has historically not done that, even in states where they have a supermajority (like CA), and even during times when they've held a trifecta at the federal level.