Re: you just need to ask AI to sum up the numbers
I'm not sure it was meant as a serious suggestion. However, if you don't know enough to write your own sum function, asking AI to do it for you will generate something that might work or might not and you don't have the expertise to figure out where its problems are. If it produces something that crashes immediately, that will at least be clear, but if it works on some but not all inputs, reading the code is the most reliable way to find that out before you rely on it. As such, asking for a program that you can't test is no more reliable and potentially more dangerous than just asking for the answer.
This is even more the case because the spec for the program was vague as always. Summing numbers isn't that hard (except for precision, overflow, speed, or various other more advanced cases), but getting them out of some system, into the program, and the result into a desired location is trickier. If you ask anyone, human or LLM, to write a program that sums numbers from a spreadsheet, most of the code will be related to getting the numbers out rather than summing them. With more potential sources, the program the boss asked for basically can't be written without clarifying where the numbers are going to be, in what format, and where the result should go.
So even though it was meant as mockery, asking for the result at least eliminates all the parsing and retrieval code, which means that lots of possible bugs are not going to be there. As LLM companies improve them, straightforward mathematical tasks will improve. That's not because LLMs will become more intelligent. It is because these companies like to hide the places where their models screw up, and this one isn't the hardest to handle properly. Figuring out whether a calculation is needed and then running that calculation outside the model is already done in some products. It's fragile, so if you need anything complex done it's likely to stop working, but for simple calculations of the kind you could quickly do in your head or on a pocket calculator, it means fewer embarrassing errors.