The original one was built by one intern[1], so it's quite possible so was this one. Not a huge resource suck when you are delegating one person to the job who just wants to learn.
Solitaire for Windows 8 (which forms the base for Solitaire for Windows 10 and iOS) was contracted out to Arkadium Games and then Smoking Gun Interactive. The team that built it was rather large, far more than a single person who just wants to learn.
This shouldn't be that surprising though, as Solitaire is in many ways a "flagship" app, which means it has to work on every single device that supports Windows 10 including tablets and phones and 35+ languages including right-to-left languages.
[1] https://www.reddit.com/r/todayilearned/comments/3zfadv/til_t...