While in your first example I would use von instead of über, especially in written language, this is a subtle difference, and I doubt that in everyday spoken language many people would notice this difference (or correct it). If they did, I would consider it as very picky.
Wovon man nicht sprechen kann, darüber muß man schweigen.
Worüber man nicht sprechen kann, davon muss man schweigen.
Although the difference is subtle, and the second sentence sounds fine, too, the wovon indicates that he speaks about (roughly) "that which you cannot define".
I think the von in the context of your example is mostly used to indicate that you give a definition of something, e.g., in your first example the meaning of Lebensfühungs-Krankheiten is defined in the first sentence.
If you use über it's a more general statement that you are talking about something.
Now, we talk about Lebensführungs-Krankheiten.
Here, you have to use über and von would be wrong (I don't think that the you third example that the native speaker provided is useful to make the distinction clear).
Jetzt sprechen wir über ...