Anyone with just half a functioning eye will notice that in the depicted
brace position the head and knee are aligned vertically. Which means that it is very much a position that is possible to adopt. In case it has escaped the
notice of any of the resident sciolists, the back of any seat is designed to be
pushed forward with minimal effort to accommodate your inflated noggin should the need arise.
You are indeed correct, assuming you're lucky enough to be on an aircraft with the seat rows so well spaced, not many of those these days. Of course you will need to undo your seatbelt too.