A professional tool is something that provides reliable and replicable results, LLMs offer none of this, and A/B testing is just further proof.
The author's complaint doesn't really have anything to do with the LLM aspect of it though. They're complaining that the app silently changes what it's doing. In this case it's the injection of a prompt in a specific mode, but it could be anything really. Companies could use A/B tests on users to make Photoshop silently change the hue a user selects to be a little brighter, or Word could change the look of document titles, or a game could make enemies a bit stronger (fyi, this does actually happen - players get boosts on their first few rounds in online games to stop them being put off playing).
The complaint is about A/B tests with no visible warnings, not AI.