You're misunderstanding what optimize means. It doesn't mean "do these things and you'll get a legendary" it means "do these things and your chances of getting a legendary are better than if you didn't".
Basically we all have around a 1/10000 chance of getting a legendary from any content and the more things you do (get caches, loot bosses etc) the chance to get one goes up a tiny tiny amount. Time played also increases this chance. So eventually after around a week of actively playing, your chance goes from 1/10000 to 1/5000. 2 weeks played gets you to 1/2500, 3 weeks in its like 1/1000 and so on. (Actual numbers pulled out of ass, just used for establishing the pattern)
The devs had said that if you play actively for around a month, you're pretty much guaranteed to have one drop for you in around that time frame. (outliers do apply naturally)