This does sound like good news, but there's a problem...

As the Twitter message mentions, people that have held their 'shares' for 12 months or more can now sell.
I cannot find how many BTC Grayscale holds, but it can never be a lot.

This makes for a problem, as the volume can never be higher than the amount of shares/BTC that has been held for more than 12 months.

This volume is probably not even 1% of the daily traded volume on current exchanges, which means at the moment this is still useless to institutional investors.