@natalie and @Linda covered this perfectly. as a side note this is literally been one of the most difficult tax seasons for all accountants out there. We've had the stimulus we have had PPP loans EIDL loans, our phones have been ringing off the hook with questions about it people who aren't our clients are asking for help so chances are if they were cold or distant all they might just be a little stressed out.
As for the due date I can tell you my firm we default our due date to the latest the IRS allows us to pay this is simply because most clients do not want to pay prior and setting up and auto debit allows them to keep the money in their bank longer.
This year we've noticed several returns get hung up in the IRS system and just saying processing I have one that we filed on February 15th that is still saying processing and we can't get a hold of anybody to find out what's going on.
You are making an assumption that the assistant dealt with everything they may deal with communications for the accountant so that way they can work on physical work on tax returns.