Your statement is exactly right and strengthens my argument. Switzerland has absolutely fabulous public transit while having low density. Therefore, the argument that you have to be incredibly dense to support good public transit is false.

Switzerland has excellent public transit. Therefore, if there is a density threshold to offering good public transit, an area would need to be at least less dense than Switzerland in order to argue that it isn't possible.

The SF Bay Area is denser than Switzerland. Therefore, the reasons for not having good transit are not to do with density, but with something else (politics, zoning, public support, etc.). But density is not the issue.