I've never heard of that 18 hour rule. It's funny because I've read a lot about fasting, but this is news to me, and it seems like this is reported from multiple sources. But I can't seem to find any authoritative references.

Autophagy is always active, the real question is how that activity changes during fasting. The number of 18 hours is fairly arbitrary, as even after a few hours of fasting autophagy increases.

Depends on what you're eating... if you're on a keto (fast mimicking) macro level intake, you'll reach autophagy a bit faster. Heavy exercise/resistance/lifting can also help speed this up, which can also lend to timing of exercise and food intake.

I’ve been looking for a good source but haven’t found one. Has it not been studied in humans yet?

Autophagy is not a binary thing. Every body does autophagy all the time, it's the intensity/volume that increases in longer fasts.