Obviously? Why do you think it would be any different?

Not sure myself.

There's more variety of content on youtube, whereas I think TV sometimes caters to lowest common denominator (although that's less the case in the era of streaming: TV is much higher quality than it was when this study started in 1987).

On youtube, you could watch lots of silly dancing videos, car crashes, etc., possibly rotting your brain. Or you could watch history lectures, book reviews, documentaries, etc.

The type of content, the length of content, the method of ingesting the content, the different motivations/precursors to TV watching vs online watching (since the study doesn't determine causation only correlation) lots of things.

Science doesn't work by looking at things and saying what's "obvious" or "common sense", because what's supposedly obvious or common sense is often actually false.