Earlier quoted context omitted.
I don't get your point. Web tools have been doing A/B feature testing all the time, way before we had LLMs.
This is very different from the A/B interface testing you're referring to, what LLMs enable is A/B testing the tool's own output — same input, different result . Your compiler doesn't do that. Your keyboard doesn't do that. The randomness is inside the tool itself, not around it. That's a fundamental reliability problem for any professional context where you need to know that input X produces output X, every time.
Not to mention that of course everyone A/B tests their output the whole time. You've never seen (or implemented) an A/B test where the test was whether to improve the way e.g. the invoicing software generates PDFs?