mynd just means ''going' or 'to go', whereas 'Go!' requires the command form Cer! or Ewch!
This particular item is a zombie - it has been deleted but has yet to disappear. Other versions put the command 'go!' into a context which makes it clearer that the command form is what is required.
dos! is another alternative which could be used here.
I do not know if it is in this particular sentence as an answer - as mentioned above, it is a zombie awaiting a bug fix by Duo before it actually vanishes.