So, how does that show that voting for Stein is the same as not voting?

Duverger’s Law just explains how the system works. It doesn’t tell you who you should vote for.

You might not like that FPTP results in this, but that’s how math works.

Again, using the same logic, a vote for Harris in a blue state is the same as not voting.

“The math” says that it’s impossible for my state to be anything but blue.