

“illegitimate” is different than “illegal”. In some context they are the same, but “illegitimate” can also mean “against rules” or “against custom”.
The President nominates these officials, but they take office “with the advice and consent of the Senate”. I take that to mean that after the President consults with the Senate, he makes an appointment, and the Senate is obligated to give an up-or-down vote on it.
I remember that time well, and Merrick Garland (yes, that guy) was the compromise candidate that Obama picked because he had the respect, of a lot of Republican Senators. The Senate had a constitutional duty to put it to an up or down vote, which Mitch ignored, because he would not like the outcome. (If Garland didn’t have any Republican support, after all, they could have just held the vote, and voted it down. But that would have given Obama another chance to nominate someone else.)
There is no law that says “the Senate must act within this period of time”. We can’t haul McConnell to the brig because of it. Yet, it is clear to me that he flat out ignorea something the Constitution obligated him to do. It can be clearly seen as illegitimate, especially when he changed those rules a few years later.
When the President does it, it’s not illegal…