Not true for everyone because Bruce Lee was a prolific multi-tasking, known to watch television, while lifting weights, and reading a book. At least according to people who knew him personally this is true.

I would say it depends on the person and their natural tendency. Most people are not good at multi-tasking but there are definitely ways. If you're watching TV you can also for example be working out and these activities go well together. If you're listening to a book you can also be chatting in text, as you can listen and write, allowing you to squeeze some social time while you're researching.

On the other hand don't try to drive a car while texting, these do not go together.