[Book3] Only change the denominator PDF value can also converge to the correct answer?

In ``6.2 Using a Uniform PDF Instead of a Perfect Match``,  the scatter ray is sampled in Lambert Distribution in ``rec.mat->scatter()``, but the pdf value is set to ``1/(2*pi)``. I think it is against the Monte Carlo Theory. Can It still converge to the correct answer?  As the book said `` This will still converge on the correct answer``. I'm confused about this.