I'm the hiring manager for Army civilians in my unit (joint unit under the Army and I'm a Naval Officer), the way things work is that in order to post the position we need the letter of intent to resign. Until we get that letter, we can do absolutely nothing to start the process. The letter, however, does not mean that the person has to resign, they can change their minds. Which is why we are only authorized to overhire for 2 weeks (at least for my unit, not sure if that is a Army standard).
I currently have an Army civilian who intends to retire in October 2021, he is adamant he will retire then, and submitted a letter of intent to resign. We know that he has a lot of leave saved up, he works with finances and will be absent just before change of FY, and his position is only one deep, so we requested from the hiring authority to overhire the position for 90 days and were denied. We were only authorized a 30 day overhire (rather than the normal 14 day overhire) due to the fear that he would not resign when he said he would, and we would be stuck with two people in the position.
Understand that this is my experience in the Army civilian hiring system and my understanding of how things work, and always contact your CPAC/command HR to determine if your unit does the same thing. I know my CPAC supports many different units, but that my command influences how things work. CPAC can usually tell me the full limits of what can be done, however, my organization usually has stricter rules about what we are authorized to do.