The nature of this downtime is quite severe. How are Googlers resolving this if their internal communications are down?

There are other ways to deliver packets... https://en.wikipedia.org/wiki/IP_over_Avian_Carriers

There is IRC as backup

I recall that their SREs use IRC?

Yep. At least some of the teams do.

not hangouts? allo? duo?

I don't think Allo and Duo are products designed for use in teams/organizations, or marketing themselves as such.

Good ole phone calls is my guess

I can't login to anything via Google Auth this morning.

It reports error that my Google Account can't be found. I thought my account had been deleted!

This is bad opsec