the code of military justice says that personnel can only reject "manifestly" illegal orders and specifically tells them not to bring their own judgement to that question.

That's a really high bar.
It remains illegal to council soldiers to disobey orders too. There is a specific law against it. There's no need to "remind" soldiers something they (as you say) are taught very early on. The Dems are doing so to lay the groundwork for insurrection. That's treason.