The challenge here is if their English skills are so poor that they need a ML model to help make it readable, their English skills are likely so poor that they

The challenge here is if their English skills are so poor that they need a ML model to help make it readable, their English skills are likely so poor that they will not spot grammatical changes that adjust the meaning of the work. That's a problem when the next person finds that paper on Google Scholar and cites it - you get a telephone game in the literature. It's hard to remove bad "facts" from the literature after a few people cite it, since you also need to go after a whole chain of papers, hoping their journals are willing to issue a correction. God help you if it's in the intro section and the authors complain because of academia's "corrections are bad" mindset. Anything that increases the BS rate of the literature is a bad thing. Also, if you go to contact the author or view a talk, and suddenly their English is broken and completely different than what they wrote, you start questioning whether they wrote it or paid someone to without disclosing it. IMO it's worth disclosing for many of the same reasons you disclose funding, ethics approval, author contributions, and conflicts of interest at the end of an article: it's metadata that protects your credibility as an author by showing that you're not doing anything underhanded.