Although California Senate Bill 420 may have provided an official, public link between marijuana and 420, the bill did not create it. 420 is a piece of slang that has been used in its modern usage since the 1970’s. The CA Senate Bill is from 2003. 
420 has contested origins, but it is definitive that nothing so recent created it. Furthermore, it is more likely that the legislators of bill 420 were aware that 420 was a celebration of marijuana, and therefore used that number for the bill as a sort of harmless, quiet joke.