That's the story that we have as the public - Slack started as a chat feature within another project. Similarly, Youtube's supposed origin story is that it was

That's the story that we have as the public - Slack started as a chat feature within another project. Similarly, Youtube's supposed origin story is that it was the video part of a dating app. At that time, online dating apps were being funded by VCs (OkCupid and even HotOrNot may have been still going strong) so it makes sense. The primary point should be that to find unicorns investors should be looking for good founders and teams first and products second fundamentally because products are flexible and plentiful while talent and ambition in the same body is not. This is what I think YC is also attempting to do.