Yeah, I don't really understand it either. There's two spots that feel that way.
The first is the spot where you jump in and the tar starts rising. It's like, you're supposed to swim up with the rising tar, but it's pure chance whether I make it over the cliff.
The other one is later on, where you have to hop across a bunch of platforms with giants and tar slugs all around, but there's this completely unjumpable gap in the middle of the platforms, so you have to just... Crawl through the tar for two full minutes.