Corona in Excel sheets.
I'm not at all surprised. This sort of thing happens all the time. You're told you have to set some system up in a short time frame, so you hack something together using Excel. You promise yourself that you will review that decision later when you have got time, but, of course that time never arrives. Then one day you hit Excel's limits and everything falls apart in spectacular fashion. Then you hack together a work around that makes the whole thing even more creaky (which, from what I've heard is what they've done). Rinse and repeat.
There are three ways to fix this:
1. Use tools appropriate to the job. That means a database.
2. Upgrade hardware and software so you can use a modern version of Excel (which can deal with a million rows of data)
3. Split the Excel files into smaller batches - the chosen solution.
I can understand why they haven't immediately taken option 1 - they don't have time. I can't understand why they haven't taken option 2 unless it's a bureaucracy thing: they can't get the hardware and software in time, even though you or I might just go to a shop and buy a computer and then get Excel online.