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

[LiuZengqiang]

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.