The best thing that can come from this is unit tests for Excel.
LLMs work best when they can call tools (edit the sheet) and test their results in a loop.
It's like the "value seek" thing Excel has had since forever; "adjust these values until this cell is X"
Excel doesn't have any way to verify that every formula in that 60k line sheet is correct and someone hasn't accidentally replaced one with a static number for example.
In a previous professional life, I did financial modelling for a big 4 accounting firm. We had tooling that allowed us to visualize contiguous ranges of identical formulas (if you convert formulas to R1C1 addressing, similar formulas have the same representation). This allowed for overrides to stick out like a sore thumb.
I suspect similar tools could be made for Claude and other LLMs except that it wouldn't be plagued by the mind-numbing tedium of doing this sort of audit.