What's special about Apple-device generated HEVC files that (apparently) x265 can't produce?
Well. Turns out that if you read this guide for HLS streams, it gives a bunch of details on how the actual video formats should be set up. In particular, for HEVC, it says: 


  1.10. You SHOULD use video formats in which the parameter sets are stored in the > sample descriptions, rather than the samples. (i.e., Use ‘avc1’, ‘hvc1’, or 
  ‘dvh1’ rather than ‘avc3’, ‘hev1’, or ‘dvhe’.) 


So I thought I could give it a shot. 

ffmpeg, by default, uses hev1. So if I take my previous command line and add -tag:v hvc1:

ffmpeg -i hank_vlog_orig_short.mp4 -c:v libx265 -c:a copy -tag:v hvc1 hank_vlog_orig_short_hvec.mp4

Presto, it works.