I would dig deeper and continue asking. Just saying they want a specific time doesn’t explain the reasons. The reason would matter though to decide if I would want to take actions. Maybe it’s just a matter of “that’s how we are used to it”, which wouldn’t mean much to me. Or they somehow don’t use the “read markers” properly and want to judge from the time whether they have read the post already yesterday or something like that. Could be anything.
It’s easy to make the case for relative time, especially online where people could be scattered across the globe, even on non-English communities. So having that as a default makes perfect sense. They would need extremely good reasons to have their system be better and worth setting as a default.