This is false. YIMBYs opposed the recent prop I to tax >$10M real estate sales to fund “emergency rent relief and permanently affordable housing.”[1]
[1] https://ballotpedia.org/San_Francisco,_California,_Propositi...
Instead, the >$10M real estate tax would mostly be paid by developers, not homeowners, because most property sales in San Francisco that are >$10M are apartment buildings rather than houses owned by rich people. Not only that, but it would be a tax paid twice by developers, because they have to buy the land and then sell the finished development.
The reason YIMBYs opposed that proposition is for exactly this reason. It wouldn't be a tax on rich people, but yet another obstacle to the construction of housing, both affordable and market-rate.
Separately, developers are already required to build a certain number of affordable housing units in every development.
If you require developers to build units that are mostly or solely composed of affordable housing, those developers will not earn enough money to make it worth building the units in the first place. This leads to progressives holding up all kinds of development with endless planning meetings and environmental impact assessments. The result is no housing gets built - no affordable housing, no market-rate housing. They allow the perfect to become the enemy of the good.
We need to work with developers to build housing units of all kinds, including affordable housing, rather than demonizing the very developers who are going to end up building those affordable housing developments that I think we all want.