I think their point is that 10%, 1%, whatever %, the type of problem is a huge headache. In something like a complicated spreadsheet it can quickly become hours of looking for needles in the haystack, a search that wouldn't be necessary if AI didn't get it almost right. In fact it's almost better if it just gets some big chunk wholesale wrong - at least you can quickly identify the issue and do that part yourself, which you would have had to in the first place anyway.
Getting something almost right, no matter how close, can often be worse than not doing it at all. Undoing/correcting mistakes can be more costly as well as labor intensive. "Measure twice cut once" and all that.
I think of how in video production (edits specifically) I can get you often 90% of the way there in about half the time it takes to get it 100%. Those last bits can be exponentially more time consuming (such as an intense color grade or audio repair). The thing is with a spreadsheet like that, you can't accept a B+ or A-. If something is broken, the whole thing is broken. It needs to work more or less 100%. Closing that gap can be a huge process.
I'll stop now as I can tell I'm running a bit in circles lol