I mean doesn't the file system simply mark a sector as "writeable" until something gets marked on it?

I'm more impressed that these areas of flash didn't get copied over with other data.

It happens all of the time, which is how data recovery tools work on 'deleted' data. But I doubt this particular instance was a file system bug. Probably a bug in the 30-day delete scheduling that happens by default in the Recently Deleted album in Photos.