When many policies that start in California get adopted by much of the Democratic party, even the ideas being proposed show that it is the direction at least some subset of the party wants to go down. When WSJ opinion section is covering pieces like this, it's to point out the ideas that the Democrats are floating. While it may not have passed this time, it's setting the foundation for future debate on the topic.

If you want a news side of it, here's the WSJ news talking about it during the primaries when many of the candidates were discussing it: https://www.wsj.com/articles/democrats-emerging-tax-idea-loo...