I've already given the source: it was the opinion of a medical professional I was friends with.
You can readily Google the idea and come up with multiple articles indicating that lung problems can cause heart problems and heart problems can cause lung problems. As I stated above, the heart and lungs work closely together, so it's not uncommon to have problems with both at the same time.
(Please kindly don't try to tell me that's not what I said. It's 3am here and these are comments on a forum, not a PhD thesis. I'm telling you it means the same thing to me. I've given my clarification as best I can and I didn't say "primarily," I said typically, which really doesn't sound the same to me. Given how the heart and lungs work, I think my friend was likely correct that the direction of cause and effect typically starts with the lungs and goes to the heart from there, but not always, of course.)
For me, "typically" does mean a majority of cases.
I'm "only" a Veterinarian, so I'm not quite current in the distribution of how this works in Humans, but from my understanding it's normally the heart that affects the lungs, not the other way around.
That statement might be true inside a specific context, like infections, maybe.
To me, primarily suggests something like "we have hard data showing that 90 percent of cases absolutely start with the lungs" and typically is more like "I have professional experience and years of observation without collecting hard data suggests that this is true at least 51% of the time."
So they don't parse the same for me, though I can see them parsing the same for someone else.
The lungs serve as a filter for everything you breath in. Modern air is generally pretty polluted. I imagine that everyone's lungs get de facto pretty gunked up these days, like the filter on a home AC that never gets changed and then we wonder when other pieces of the system start showing strain.
I imagine it frequently goes unrecognized as starting in the lungs, but I think it probably does. Please note that "starting" there also doesn't preclude other contributing causes, which is another reason I object to the word primarily. Saying "First X happens and then there tends to be a cascade effect from there" absolutely isn't the same as saying "X happens and X is the entire cause of the problem with zero other factors contributing to it."
Edit: I will add that I believe it starts with the lungs in part because reading up on altitude sickness did wonders for my understanding of my condition which has substantial gut involvement. When there is a defect in air quality, it rapidly starts impacting other systems, like the gut, liver and kidneys, and there are huge knock on effects that can be outright deadly.
Before reading up on altitude sickness, I had this hand wavy idea that the gut and lung issues are related, but afterwards I had a clear and definable connection and that connection is how the body processes blood gasses. So, obviously, the connection is the circulatory system, which is powered by the heart.
You can readily Google the idea and come up with multiple articles indicating that lung problems can cause heart problems and heart problems can cause lung problems. As I stated above, the heart and lungs work closely together, so it's not uncommon to have problems with both at the same time.
(Please kindly don't try to tell me that's not what I said. It's 3am here and these are comments on a forum, not a PhD thesis. I'm telling you it means the same thing to me. I've given my clarification as best I can and I didn't say "primarily," I said typically, which really doesn't sound the same to me. Given how the heart and lungs work, I think my friend was likely correct that the direction of cause and effect typically starts with the lungs and goes to the heart from there, but not always, of course.)