Agreed. But you do not have to maintain your other status while in TPS. You maintain other status because at the end of TPS, you want to fall back to your original status and continue to reside in the USA without exiting.

In this case, his parents want to return back and before the end of TPS. So his parents are free to leave. And I do not see any issue coming back as their presence here in the USA is not consider unlawful. Their stay was lawful via TPS.