Proving my exact point
I am completely aware it's capable of general purpose coding, but even if you give it a manual it is not capable of getting all the neccesary contect(like seeing if the device actually has a GPIB port) I have naively tried this exact thing and only ended up in a world of errors and having to go back and learn it anyway. I'm also aware it will get better, but that isn't right now is it.
Yes, it pretty much is right now. But it's like the Sorcerer's Apprentice sketch in 'Fantasia.' You are still programming and you can't let yourself forget that. It will do what you tell it (eventually) and it will do a good job (eventually), but supervision and validation is still your responsibility.
The model doesn't know or care if there is a 'real' GPIB port on the power supply, or if it has to be added externally as an option. It is not fair to expect it to. From my reading of the manual (and Tony may correct me here), if you add the option, it does have a perfectly legitimate GPIB port. It may not be a good way to get the job done, but that's your call, not HAL's. Not yet.
Getting back to the OP's problem of controlling an older DMM, that is actually an easier problem, since unlike the power supply, the DMM won't fry anything if misprogrammed. I don't have one of those here, or (again) one of the AR488 adapters, but I have no doubt that if I were to, say, tell Claude or Codex to write a program to control an HP 3478A from the same era, using (say) a Prologix adapter which is very similar to the AR488, that it would only take a few minutes to get it working.
I also have no doubt that if I just went to the freebie chatgpt.com page and told it, "Write a program to control <whatever> with an AR488", the result would be comparable to what you got when you (apparently) did the same thing. Again, it reads like English, and it is English, but it's still a program, and has to be treated as such.
Even the Claude/Codex version probably won't work the first time, but guess what: that would also be true if you or I had written it. I agree that the free ChatGPT output may not be worth bothering with, or arguing about.
So there is still a lot of competition for best tooling.
Opus 4.6 is a step change over Sonnet. I don't have enough experience using Codex/Copilot for programming to say how it compares to Opus 4.6, except to say that I was impressed with how 5.4 Medium handled the modification to your code above, but I do have enough experience with Opus 4.6 to be favorably impressed with it.