It's very common for hardware vendors and hardware teams to have extremely long email chains where they copy everyone involved for the length of the project. I imagine that the vendor copied the old Google colleagues by mistake.

Misdirected emails happen. I was even recently called by a vendor on my cell phone (side effect of open offices: no office phone) for an old company that wanted to talk about the sensors they were using for a new project. The vendor still thought I worked there even two years after leaving.