I see both of your points and I get how compelling the second one is, but what can I tell you? It was definitely already in progress and it has to do with long-term leadership changes inside the organization and thinking about what YC is really supposed to be. Keep in mind that Garry's previous years at YC were before the late-stage business got started. Why they made the announcement yesterday, I have no idea—perhaps someone enjoys Hacker News drama!
Btw I don't think the difference is spin, I think it's the opposite actually—from my perspective (and just to be clear, I'm not privy to the inside discussions) this change is not a reaction to short-term business climate, it's a vision thing. Most people underestimate the power of having just one thing to focus on and knowing exactly what it is. It's a cliché everyone pays lip service to but almost no one really does. It was a huge insight for me that running HN is all about focusing on one thing*—it's a kind of superpower that turns a surprising number of hard calls into easy calls.
* https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...
And as for assuming good faith that this was a fait accompli already in progress... I'm sorry, but I can't get past "We're doing this on the first business day following the biggest upset to our industry in a decade and a half. But that's just a coincidence!"