I'm almost not using AI, but a possible scenario is that the contributor spend like 20 hours in total.

Something like using the AI to get an initial bad version, make some tweaks to the prompt, make some manual fixes, ask the AI to fox something else, noticing some new related feature and asking the AI to add it, making some benchmarks and deciding to remove a small feature, or perhaps deciding between two similar implementations, add a few more manual fixes here and there, run the extended version of the automatic test and find a weird bug in the unusual setup, make a few fixes with the AI and manually. So after 20 hours of work, the final version has only 50 lines that have been rewriten like 5 times each. Now the mantainer can review only the final version in 1 hour or so.

This is very different to spending 5 minutes asking the AI to write a patch, that has 1000 lines that does not even compile and sending it to the maintainer without looking at it.