PRs are never merged because the github repo isn't the source of truth for Flow, their internal systems are. They have this workflow where they will import PRs from github and apply them internally, then close the PR when it's done.
At a glance, it looks like all PRs are just closed, but looking at them individually shows a different story.
It's the same with React-Native, and several other OSS projects by facebook, and Jamie knows that, having worked at FB when that workflow was used.
It has its faults (I personally really hate the workflow they use, but I get why they use it. Github is where the people are, and Phabricator and other internal tools are difficult to integrate into it in a lot of cases, but I still hate it), but to say it's because they don't care about OSS or they do all their work internally isn't true. Plenty of FB employees and outside contributors both make PRs on github, discuss them on github, and just merge them internally closing the PR on github.
Flow has plenty of issues without needing to make them up or exaggerate them. Their errors still take weeks for many to understand and get used to, flowtyped is a mess compared to typescript's type distribution system, their fucking habit of single character type variables makes looking at built in type definitions extremely difficult, the flow binary still is extremely unstable on windows even after years, and the recent move of aliasing Object and Function to `any` seems like a misguided attempt to simplify some code and speed up some checking.
But at no point do I think they should can the whole thing, and I absolutely think they care about open source. Like I said in the comment above, Flow is in a bad place right now, but they are putting their money where their mouth is, the Flow team is reportedly growing at FB, and over the past few months I've seen a massive uptick in the number of releases, blog posts, external contributions, performance, and a significant decrease in the number of bugs I was hitting on a daily basis.