I would guess it's not that simple.

People's reaction are varied, this tweet actually resonates with people's beliefs much more strongly than what you might expect. For a lot of people, medicine that you need to continue living should be free. So I would say when this went viral, people at the very least go: "hmm...".

Couple that with reactions from politicians and suddenly you find yourself in a situation where the price of insulin is once again being questioned.

So imo, it's not just a funny tweet but more like a conversation starter and so it absolutely makes sense to me why this would be reflected in the stock price of insulin makers as a whole.

I have no idea if the tweet was actually directly related to the price drop and lean toward being skeptical of it, but I don't think those other companies also dropping is great proof of that.

Assuming the tweet was believed to be real (by enough of the right people to cause this sort of stock move), the expected outcome would be drops in those other companies as well, since Lilly giving insulin away (the tweet was: “We are excited to announce insulin is free now.”) would obviously cause a large impact on their ability to continue to sell it.