Right now, the game allows you to enter anything you want, but you can always self-restrict to make it more interesting/challenging. There is an ongoing modification, currently only available in the debug mode, that checks input against the OJE dictionary, but it only just notifies for invalid joseki input, and does not actually restrict it.
Sorry, this is a weird design choice. If the solution is only X moves long, but if someone enters in a longer sequence where the first X moves correctly match, then the game ends into a “solved” state, by disabling the submit button, showing the share buttons, and giving the text “Good enough!”.