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Incorporating LLMs into software is a software dev job from what I have experienced. You don't need to know anything other than how to design and integrate APIs. So, the real answer to your question is dependent on what companies are actually "building" something versus which are simply slapping together a bunch of things (I am not against either, if my phrasing suggested otherwise!). A lot of companies and research groups are emerging where you do need ML theory and hence ML engineers. Take for example PPFL (Privacy Preserving Federated Learning) or similar fields.