Yes, they are. Especially given that not a single method of generating scores guarantees at least two 15s. And not a single line in the DMG supports your reading of the PHB.
Look, you don't get to say you can
only read the PHB or
only read the DMG. That's not how the game works. You need to use the books together. That's why they keep referencing each other throughout.
You're just saying, "Well, it doesn't say Gary is going to send Pinkertons to your house to confiscate your books if you dare play without two 15s." Congratulations. You have moved the goal post so far that your point is defensible. You've discovered that Gary will let you play the game however you want, including ways he might think are not going to be fun to the extent that he specific points it out that it's not a recommendation.
That doesn't mean the books aren't
advocating against doing something they very plainly state is not a very good idea or not conducive to a fun experience. The PHB says, "We suggest that PCs have two 15s or better because the PCs are supposed to be exceptional and they're going to need it to survive." And then in the DMG he says, "Technically, you
could use 3d6 straight, but the reality is that that not much fun for most people. You'll never get the play the cool classes, and some players will just get frustrated. So here's four other ways to generate stats, none of which are 3d6 straight." You really can't argue that 3d6 straight is "playing the game the way it was intended." You're clearly
not by any interpretation of what's in either the 1e PHB or DMG.
If you're comparing it to how
Basic stat gen works, that's not really a fair comparison. Bonus in AD&D start at 15. That's not like B/X. In B/X, they start at 13, which is
much more common on 3d6. On 3d6 you have a 26% for a 13+. On 4d6 keep 3, you have a 23% chance of a 15+. And even OD&D and B/X typically allowed you to adjust your prime requisite ability score.
So, going back to
@Hussar's point, showing up to play AD&D with a Fighter with 17,10,9,15,16,12 should really not be very unusual.