I'd say you can learn enough from any medium if you try hard enough. However, just "watching" those vids won't suffice. You have to practice (= write code/complete a project).
> Can you learn enough from mainly videos to be employable as a web developer/software engineer?
Probably, though mostly books/online text is probably better. (Or, really, no, you can only do it by mostly doing web programming, but you need some external resource, and either text or video [among other things] -- or a mix -- can serve as that external resource.)
I also believe that it is. There is so much data contained within video that it's impossible not to obtain something if it can be presented correctly. The trick with coding videos is grabbing the users attention, not losing them, and showing them the results they can expect. If they're not engaged, they're learning. With so many subsets it can be hard to achieve engagement, People want to learn different things, but everyone needs to learn some things. The most efficient way to learn some things is not necessarily the most efficient way to teach some things.