Isolated Selected Group

I would recommend that you get a good book. Classroom in a book or Real World Illustrator (yes it's old, but it's still good). Then read that. Just diving into random videos will probably not help you with your learning.

As for that video: I don't think isolating makes a lot of sense in his workflow. He could have just ungrouped the thing.

Did you follow along with the presenter? Did you actually do what he did? Because then you should notice if something is necessary.