1) There's a Francis Darwin quote that is relevant "In science the credit goes to the man who convinces the world, not to the man to whom the idea first occurs." I hope to explore that idea in an essay someday, for now I'll just share the quote. I am not trying to make a prediction here. Getting into YC is not convincing the world. No one has done that yet in this market. I know some of the people involved in Focus Mate and absolutely think they could be the #1 player, but assuming a big market is there it's certainly game on and who was first is a rounding error and the rest comes down to execution. It's a wide open field.
2) I did YC with AirBnb and at the time CouchSurfing.org was indeed a decently known thing in some circles. For every YC investment I'm sure you can find multiple "really good implementation of the concept".
3) This is still very much early innings in this space. As an investment, it's very hard to predict whether Flow Club or FocusMate or one of the other 8 will be the big unicorn (if any). A lot of it will come down to the founders. Even if there does turn out to be a huge market, some may take an early exit (Facebook v. MySpace, for example). As an entrepreneur I generally opt for the small earlier exit, but as an angel investor I try to avoid investing in people like me (both to hedge and b/c of power laws of returns).
4) A friend had brought me to SoulCycle once and so I totally get the "Peloton for Work" angle. So that's what led me to give Flow Club a shot. And I have found it very productive a couple of times per month. The coach and music have been great. For me the FocusMate style was intriguing, but it was just slightly below the threshold where I would give it a shot. Could those be 2 different markets with one being 100x bigger than the other? I have no idea but seems small things like that could actually make a big difference.