And such a conversation would still be classified. If disseminated within the US government without classification and unmasked and no warrant against Flynn, it would be illegal.
Open source does not mean unclassified.
I’m certainly not an expert on this so if somebody with more experience wishes to chime in I’m all for it.
I was once in a discussion where a guy from my organization had pasted a definition from a college textbook onto a slide and while it was on screen another guy said 'that's classified!' I objected, said it's public domain knowledge that I and thousands of other engineers learned in college. I was told it was classified in the '50s and never declassified so it's still classified. They couldn't wipe it out of textbooks but they could force us to wipe our computer hard drives for having that well known definition on them. And they did. That sucked.
So yeah, classified isn't about how you got it or even if it's already public, it's about what the content is.
From the time this saga began back in early 2017 I’ve had a sneaking suspicion that the Washington Post and NYT got the information about the Flynn surveillance from a source outside the U.S. government.