Alexa picked up you saying Alexa, then heard the Google's compliment, which it mistook as having come from you. It wasn't responding to Google, nor did Google activate it.
You're probably right in the first half, but I wouldn't be too surprised if interactions between voice assistants are special cases that are accounted for by the programmers as a little bit of an Easter egg
They probably interact more through zeroconf/avahi/mdns than speakers and mics. I think both devices have some implementation of blue tooth low energy network mapping. They know about the other smart devices in your home.
There was a Superbowl commercial for Alexa that sounded funny. Every time they said Alexa it sounded robotic. I guess they scrambled the voice just enough for the real units to not respond. Also, the first time they said it... It was subtle, but they said "Lexa" and not "Alexa."