The article or brief didn't give us the detailed data on foot impacts or how that blood flow is qualitatively different from maximum blood flow during any strenuous exercise. I.e. not sure if the "back-flowing waves thru the arteries" that occur and provide the benefit of "dynamically regulating blood flow to the brain" can be achieved in other ways. I'd think if its all 'blood flow' swimming would be among the better and similarly road biking can be - its all a matter of where and how hard you go.
You bring up an understandable question. Brain health has many question marks. I see studies that focus on the amount and intensity of exercise in relation to preventing cognitive decline and no distinction is made between types, leading to the conclusion that any type of exercise should fit the bill just as well. Yet I also see studies comparing different types of exercise and distinctions are found. In general, biking and/or swimming seldom are shown to slow cognitive decline while dancing is the one consistent "winner." But this brings up question marks as well. Maybe foot impacts play a role, maybe balance plays a role, maybe the social aspect of dancing is really the key ingredient. Also, when I see these studies comparing different exercises on cognitive decline they are usually with older cohorts; the dancing of these older cohorts may inherently be more strenuous than the bicycling or swimming they are asked to do. Like I said, many question marks.
One reason for so many question marks is the multiple factors involved in living well. Positivity, sociability, activity and exercise seem to be all very important. Also, things like hugs and laughter release hormones important for brain health while stress releases hormones important for temporary "fight or flight" but detrimental to brain health. Thus, you could get the absolute "best" exercise possible (whatever that is), but if you don't have all the boxes checked you are at greater risk than someone who gets good exercise but not the absolute "best" (whatever that is). I suspect why dancing consistently does comparatively well in all cognitive decline studies is that it checks boxes that doesn't have to do with exercise. I'm not an avid dancer myself so I'm definitely not trying to push my own agenda on someone.
My own position on brain health is to live well in all facets. If you check off all the different boxes, and perhaps most important of all in our society is to get rid of chronic stress, then you need not be concerned about finding the "best" exercise, or "best" activity, etc.
On edit: In regards to the mountain biking v road biking (I've done both, though more road biking), studies have shown that some anaerobic spurts are beneficial, more so than going for a longer period of time without them. I've pushed myself to anaerobic levels while road biking, because that's the type of person I am, but such anaerobic levels are unavoidable (no pushing necessary, it has to happen) if you are mountain biking over very steep terrain and I still would give that the edge.