The origins of heather may determine its density or weight, but not its taste.
I agree with everything else you said, but this particular notion is a subject of some debate. There are plenty of folks with palates far more experienced than mine that I’ve heard debate that briar from different sources can actually produce slightly different flavors. Among them are prominent pipe makers and internationally known tobacco blenders.
Now, whether that is because of how soft it may be, it’s environmental surroundings, how it’s processed, etc. I have no idea. But it certainly ain’t sap, hahaha.
In my own experience I’m inclined to agree with you, I have yet to taste any notable or consistent flavors that I can say with certainty correlate to a certain briar source. And to me, the impact of other extraneous variables (tobacco type, dry time, cadence, packing, etc) seem as though they would far outweigh the impact of briar origins, at least to a palate with my sensitivity and/or experience level.
Just pointing out that this is indeed a subject of (it seems legitimate) debate among those who can certainly speak with some authority. But not a one of them would tell you it has anything to do with sap, hahaha.