No, it is not a myth. It is *economically* locked in. Technically, you can move it, but you lose your master node, and its reward. So you won't.
Every coin in every wallet of every crypto where people are holding and not selling is "economically locked in".
No. At every instant, with a non-interest bearing token, you CAN sell it from the moment you think it will go down. If you hold it, you think it will go up. With an interest-bearing token, you include the interest in your calculation. You only sell if you think that the loss will be higher than the interest it will bring when you do not sell it.
Moreover, if most of the interest-bearing accounts are in the hands of the same entity that manipulates the market, he keeps it locked in to increase his lever on the market of course.