And no, your eyes and memory are not deceiving you - Redstone 2 hasn't even shipped to Release Preview ring phones, let along production devices, but there's a distinct chance the two OS branches might overlap in time here!
Microsoft's piece included:
Today we are excited to be releasing a new build from our Development Branch! Windows 10 Insider Preview Build 16170 for PC has been released to Windows Insiders in the Fast ring. As we mentioned earlier this week, you won’t see many big noticeable changes or new features in new builds just yet. That’s because right now, we’re focused on making some refinements to OneCore and doing some code refactoring and other engineering work that is necessary to make sure OneCore is optimally structured for teams to start checking in code. This also means more bugs and other issues that could be slightly more painful to live with – so check your Windows Insider Program settings!
So build 16170 (and beyond) is the new baseline for Windows 10 Mobile on the Fast ring as we head into Redstone 3 territory. Will this be the last 'great' W10M branch before it's all folded into full 'Windows 10 on ARM', complete with a new shell/UI and new hardware required? Time will tell, but your 'old' Lumia 950/XLs etc have got at least one major new branch of Windows 10 to look forward to, it seems.
In terms of timing, Microsoft needs to give all Windows phone users a good day or so to see the message and change their ring if needed. I'm guessing that we could see the first Redstone 3 builds for Mobile sometime next week.