I don't think we can say it was clearly anything yet. They've been working on these for a long time. And whether people like it or not, Valve has a very public philosophy (if you've ever watched their Steam Dev Day videos): "People don't want to hear emtpy words. It makes them angry when you speak and fail to deliver on your promises. Speak only when you have something to show them."
This is clearly a tactic to build hype. And if they're actually launching in May, it's a good tactic. Drop a tiny scrap, let the community get invested and speculate away. Then when that's run its course, drop some more. Keep them in a frenzy and build the hype till you launch in May.
If we're talking counter-measures, I half wonder how much the industry has actually known about things. Valve's been running the factory for quite some time now. Facebook has to have a general idea of what's up. I wonder if the Rift S was Facebook's counter to the fact they knew Valve was launching soon. And they needed a new product to be a able to sell as an ugprade. So they Franken-smashed the Go and Quest together as some cheap new product they could counter it with (on the budget end).
True, the Rift S could be the Oculus response to Index, trying to get ahead of any Valve momentum. Either way, the timing of these announcements doesn't seem like an accident. They're all competing against a not-huge PC VR market and need to get as many people to their ecosystem as they can early on.