"You guys are reading too much into this... There's no way that any reasonable person would consider that an official act and this hypothetical president would surely be swiftly impeached and convicted. You're worrying over nothing."
- Roberts, probably
The payment by Cohen to Daniels for her story happened before the election. The problem is that a bulk of, if not potentially all of the fraudulent payments to Cohen from Trump to reimburse him didn't happen until he was president. I don't remember exactly when the repayment started.
The worry I guess is that while the crime itself doesn't involve official acts, some of the evidence of the crimes used in the trial might have just become privileged communication that can't be used as evidence. I don't think anything has changed personally reading it, but I'm no lawyer. So they might have agreed to postpone the sentencing as a bit of caution to review everything to make sure that too much of their evidence didn't just become inadmissable. Basically, it seems mostly like a bid to make sure they don't get torched on appeal.