They're giving you the advice to give the best shot at joining. It will take a few months to get the traces out of your system. Even if you show up clean on a urinalysis, you can still pop for the blood test. That's why they want you to wait. If you were a heavy smoker then you might want to wait a bit longer.
There's no underlying motive by the recruiter here other than giving you a good chance for a clean sample.
Caution: I have two brain cells. Sometimes they both work at the same time.