[also posted this reply to your comment at Ars]
This is interesting but is likely not the reason. Note that the mid 2009 13" MacBook Pro (NOT Sierra supported) and the mid 2010 13" MacBook Pro (is Sierra supported) use the same AirPort/Bluetooth board:
Mid 2009 13" MacBook Pro Airport Card
Mid 2010 13" MacBook Pro Airport Card
(yes they are using the same photo in both guides, so we do have to trust that iFixit has this right)
The 13" MacBook Pro can have quite different internals than the 15" or 17" for a given model year (e.g. in the mid-2010 models, the 15" and 17" went to i5 and i7 chips while the 13" stayed with Core 2 Duo; 13" never had two GPUs) so you can't generalize across the 3 screen sizes.
If someone can figure out the critical difference between the mid 2009 and mid 2010 13" MacBook Pros that causes the difference in Sierra support, then we might have a better understanding. The known changes for the mid 2010 are unofficial support for 16 GB RAM (instead of 8), a slightly faster CPU (albeit from the same "Penryn" Core 2 Duo family), and a newer GeForce 320M GPU (but apparently the older 9400M is in some other supported machines). Also a better trackpad, battery, and enhanced audio passing over mini-display port.