That was the original intent. They only recently added the "chatbot-y" kind of stuff since the infra is all already there. The main uses were for their translation tools and PDF alt-text generation (which I believe disabling ML will disable as they rely on the on-device transformer tools to do).
>>OneDeu+(OP)
I disabled browser.ml.enable and local translation was still working. In my case, that's all I need, but it looks like it still allows on-device transformers.