Existential types? The author alludes to this, and then proceeds in ignorance of what he'd just said for the rest of the post.

In Java, all you have for these is interfaces, so it's no wonder people think that types are problematic.

Types aren't the problem. Crappy types are the problem. Not making the types lightweight enough that people can dish them out at their pleasure, that's the problem. When the math don't work well enough, make better maths.