Lightroom Classic: Sort by edit time doesn't sort by edit time

If you choose View > Sort > Edit Time, you get a sort, it seems, by the last time that Lightroom has "taken notice" of a photo. For instance, if you Read Metadata from a photo, that will bump the photo up to the most recently "edited" in a group of photos, even if the metadata hasn't changed.

I would expect that sort by edit time would sort by the last change made to a photo (i.e., the latest of (a) the photo's modify date on disk and (b) the last edit made to the photo in Lightroom.) Unfortunately, Lightroom seems to have an overly broad idea of what an "edit" is.

I need this because I need to compare derived and source documents (edited outside of Lightroom) to confirm that the derived version is later than the source version. I thought I could use Sort by Edit Time to confirm that one file was edited after the other, but I can't. The only way to compare edit time is to use the Finder, or compare the "Date Time" metadata by hand.