Well in the realms of actually considering learning about trading, then yes, it doesn't help much. It does, however, let you learn how the trading site works, as well as some simple ideas on what you can actually see on charts. Certainly doesn't help you in actually learning when BTC goes up or when it goes down, but it does help you from suddenly making a mistake by not knowing what a certain thing does.
No. For example, if you test your new strategy under demo conditions, how the knowledge and data obtained can be false?
What data can be obtained from the demo order book or from the demo chart? Right? False. Go to any demo account and see how the order book behaves and how the chart behaves. On the demo chart, have you ever seen the fake straits? By definition, they cannot be there. Because such things are organized by the exchanges themselves when they trade against their client. And on the demo exchange, everything is reproduced in neutral conditions, without any manipulations from bots or market makers. You will not see this on the demo exchange, but in the real market, at the first encounter with such manipulation, your knowledge gained in the demo mode will not help you.
Do not forget that the demo exchange and the real exchange are 2 different exchanges.
To make an analogy on what Ratimov said, it's like comparing high school to college, or college to the workforce. In my case, it was the latter, when I entered college I basically threw away everything I got from high school and started from scratch simply because there was a stark difference between how HS hit me and how college hit me. And just like other cases, transitioning from college to the workforce isn't exactly an easy experience, heck some people can go straight to the workforce (which is imo, much ideal especially now with how you can learn everything by yourself).