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".
The world gold supply that sits in vaults with the expectation of value accrual is "economically locked in".
5, 10 and 30 year interest bearing bonds that are not on markets are "economically locked in". (Technically you can sell them but you'll loose your coupon payments, so you won't)
All the Bitcoin on orderbooks in the range above the current trading spread is "economically locked in".
The implication of the "Dash is locked in masternodes" FUD is that the supply is blockchain constrained, not "economically constrained" and therefore somehow shouldn't count towards marketcap which is not remotely the case. Here's an example of a coin which does use that approach:
