Yes the conclusion is baffling.

As you say I believe the correlation is reverse causality. It's much more likely that people who consume stuff with "artificial" sweetener are already at risk for stroke than the other way around.

If you don't have weight/cardio problems it is weird to consume "sugar-free" stuff and associated because they are almost always worse tasting than the real deal.

To have any importance they would need a big population sample and correct for already existing risks for stroke and I believe they would find that this stuff has very little impact, if any.

But as always, it doesn't cost much to limit consumption, so why not?

This doesn't apply to a study done in cell cultures.

To the reader, I strongly and vehemently urge not listening to some rando on the internet (as opposed to the scientists) who asks you to dismiss a study, because the risk-reward calculus here is strongly in favor of not taking the unnecessary risk of brain damage. The rando will not be around to look after you after you get brain damage.

I also urge the reader to not assume a lab experiment produces solid nutritional guidance.

Lots of things kill cancer cells in petri dishes that have proven to be useless as medicines.

And your repeated ad hominim "rando on the internet" is counter to good dialectic.

The stroke risk alone is sufficient to not take it.

Unlike with treating cancer, there is no meaningful reward here. As I noted, the risk-reward favors not taking it.