Hi there,
Actually Your’e half correct… LOL
If I may 
The word “Xiao” in Chinese means Tiny. The Footprint was establish several years ago. They want to develop innovative ways to squeeze more into less… Sound familiar 
To do it properly you need a cutout in the Board. The Battery Pads are the main point so, any extra I/O is gravy. The C6 and the S3 are so close in specs It’s just a matter of time before even further combinations show up from Espressif to blur the line even further.
They market the C6 as dual core , but no code for it exists, AFAIK.?
with only a 1uA in sleep difference between them, I’m going S3 unless I have to have the WiFi 6 stuff or Zigbee, matter etc.
S3 is more bang for the buc and a TRUE Dual core IMO. the PSram and more I/o via a connector is to easy to pass up. I’m banking on them combining or adding too in the future,
HTH
GL 
PJ 