I think the point is more along the lines of:

Sick leave use is generally a predictable cost item. Age seniority time of year alv equipment etc. We have decades of data; therefore it is a relatively known negotiation quantity for both company and union. The covid era may have changed that but who knows how, and more importantly how could one model the future usage (and therefore the cost basis) on an excel spreadsheet that might be used in contract negotiations?

I'm probably wrong.