If the demand drives the price higher than the NAV then the sponsor will deposit an additional basked of 10,000 BTC to the trustee and trustee will issue 50,000 new shares. The trust has now grown by 10,000 BTC and has 50,000 more shares outstanding so each share still represents 0.2 BTC. Those shares can then be sold on the market thus lowering the market price closer to the NAV. Note there is an economic incentive here as the newly issued shares only cost 0.2 BTC each but can be sold for more than 0.2 BTC.
Of course institutional investors can do the same thing so if the sponsor doesn't act then someone else will and scoop up that risk free arbitrage. As an alternative when price exceeds the NAV, an institutional investor can sell 50,000 shares short, deliver the underlying asset to the trustee, collect 50,000 newly issued shares and cover their own short with the new shares.
A nice scam.