Given r=f(θ), we are generally not concerned with r′=f′(θ); that describes how fast r changes with respect to θ

I think this part from the textbook describes what you’re talking about

Instead, we will use x=f(θ)cosθ, y=f(θ)sinθ to compute dydx.

And this would give you the actual tangent line, or at least the slope of that line.