Existentialism is anti-science. It is an outgrowth of European Romanticism. I know they might consider themselves as humanist Existentialism Is a Humanism - Wikipedia, but this is because humanism can have very general meanings.
As I said elsewhere, there are also different forms of individualism, many of them are hurtful. Same for humanism.
The point is to find which form of humanism brings to the highest goal.
And, IMO, it’s secular humanism.