With all due respect, this appears to be a prime example of HR having no idea what they're talking about. Some HR folks know our CBA exists and apply it correctly, and others... don't. I guess the same goes for ATMs.
This time is not at the ATM's discretion to allow. Per the article you reference (Art 29 sec 4), "up to 64 hours of excused absence... shall be granted" and "employees may be required to provide justification for the use of this time." A hardship is absolutely a change in official post of duty, and your ATM citing Art 99 is not relevant here.
I suspect what gets ATM/HR confused is that some BUEs seem to demand this time and overlook the "justification" piece in the article. If a BUE goes "give me tomorrow off for COS" and refuses to provide any sort of reason, the ATM probably has the discretion to deny it. It's like requesting sick leave, but refusing to provide any information why (I'm usually happy to give a play-by-play of my nasty symptoms, if management dares to ask).
If the BUE goes about it reasonably and gives justification - and the ATM still denies it - there's a good case to fight it.