I suspect that the Upwork guy, however, would realize it if he were making a tremendous error, and not just keep going.
But in any case, tremendous errors won't actually cause me big problems, because I'd catch them on my review of the P&L. Small errors I might miss (whether committed by human or LLM), but they're not going to be material anyway.
Bookkeeping is simply the systematic recording of financial transactions.
Accounting is the interpretation, analysis, and reporting on said data.
That's why an LLM is good for bookkeeping for me - I'm not actually using the books in any strategic capacity, and I know the numbers in the business well enough that I'll catch any obvious mistakes. The books are purely to give to my CPA to do my taxes at the end of the year.