Those 12" Macbook's aren't capable of driving a 5K display at 60Hz, period, even if they had a TB3 port. However, a TB3 display plugged into a standard type-C port would negotiate a DP1.2 connection so it could run 5K@30 or 4K@60.
I'm not saying they should. I'm just saying there will be confusion about there being a Type C connector on the monitor and the laptop, and that they don't work together.
But I'm saying that they probably will work together. Downgraded, sure, just like when you plug a USB3 hard drive into a USB2 port. It's not fast, but you can get your files off.
