> Seems like the folks at Stripe didn't do very good diligence
Perhaps, although given the market they're in, success for Stripe may look very different to most others investing in Fast.
Fast weren't going to create their own payments system, so being entirely or substantially backed by Stripe payments would have been valuable to Stripe once Fast scaled. Investing in them as a mechanism to encourage this could have been quite beneficial.
It's also quite possible that a desirable outcome for Stripe was potentially buying out Fast and integrating it into Stripe. At that point Stripe would already have plenty of inside knowledge, being on Fast's board, and already owned a significant proportion of the business.
Investing ~$100m to get this might sound like a lot, but if you make 10 investments like that, and get one success out of it that can generate ~$2bn of revenue at some point for Stripe down the line, that's probably around break even, assuming 50% margin. Very rough approximation, but at Stripe scale, that sort of business is entirely possible. TaxJar is another example, not sure if they were investors, but they acquired the company and it'll probably end up contributing that sort of revenue, possibly more.