I think it's misinterpreted as a recommendation, or even a requirement. I just take it for what it says plainly. The game is kind of built assuming you'll have a couple of high scores, and so if you don't have that, it's going to be difficult for the character to survive. In other words, if your character doesn't have those scores and dies, don't feel bad. The odds were against you. If you do survive several adventurers, then you're doing a great job playing!
I think if ol' Gary were real serious about every player character NEEDING two scores of 15 or higher, that the alternative attribute generation methods listed in the DMG would have taken that into consideration more directly. How hard would it be to say, roll d6, six times in order. Bump your two highest scores up to 15 if they are lower. In fact, I have a "newbie rule" where in exchange for playing a Human Fighter, you get to bump your STR to 16 and CON to 15 if they are lower. The alternatives actually listed do not guarantee the two 15's.