It’s probably not true anymore, but at the time this guy was being radicalized, you’re right, it wasn’t algorithmically catered to them. At least not in the sense that it was exposing them to a specific type of content.

I suppose you can think of the way reddit works (or used to work) as being content agnostic. The algorithm is not aware of the sorts of things it’s suggesting to you, it’s just showing you things based on subreddit popularity and user voting, regardless of what it is.

In the case of YouTube and Facebook, their algorithms are taking into account the actual content and funneling you towards similar content algorithmically, in a way that is unique to you