Well the reliability of a boot has really nothing to do with the fluid it contains. Either it has a hole in it or it doesn’t. And there’s actually no reason this couldn’t just use grease, in fact it might have to. But unlike a CV joint, this only needs to articulate in 1 axis. No rotation, no translation. Clearly this is not the complete assembly we’re seeing here however. There’s a wide range of much more fundamental mechanical problems that must be solved that this simple animation doesn’t show. There’s a number of other ways to seal an assembly like this as well. I’m very interested to get my hands on one to tear down. Maybe I’ll go patent diving and see what I find.