I saw it as a very Don way of seeing the world. His genius as an ad man (one that Peggy shares) is that he can draw from himself and relate it to the world at large. When he talks about the terrible thing that happened to the world i.e. the Kennedy assassination, he knows that only someone who has already experienced such a terrible dislocation in their personal life AND knows how to use it (as Peggy can, and as he can), will be able to communicate in the new world coming.

As for the over-subtlety. that's a very Mad Men thing, especially in the Don/Peggy relationship. Their secrets are so sensitive to them that, even between each other, they can't come out and speak them directly.

I have to add that the scene was also the first expression of true affection between the two. I never quite felt any father/daughter-like bond between them before, but when Peggy said "you'll never speak to me again," I welled up right along with her.

Bert made the sales pitch to Harry – he could join SCDP as Head of Media or go to McCann as a mid-level cog in their machine. Harry already complained to Pete in the last ep that he was going to die unnoticed behind the desk at SC. Bert pushed exactly the right button.

Here's how I interpreted Don's conversation with Peggy (YMMV):

He didn't say they were SELLING things, he said "people BUY things." As in, people buy a certain vision of themselves in the world, a vision of who they are, of how the world is supposed to work. And then something terrible happens and that image is destroyed. Parts of yourself are lost forever. He and Peggy understand that and know how to tap into that void. They can speak to the deep need inside people to make their world whole again, because they've experienced it.
That's a very rare and very valuable talent.

That was my take on it, anyway. I think the scene could have been written a little better. What's the point of dialog that leaves so many people saying "WTF was that supposed to mean?"