This isn't really true. Google Voice numbers are managed by bandwidth.com and have been taken by attackers submitting fraudulent number portability requests in the past.

Don't you have to login to your Google account to port a number?

It has been possible in some instances for an attacker to port a number directly from the underlying carrier, in this case, bandwidth.com.

When I saw this happen, Google was not aware the number was gone, so calls and texts from other Google Voice users still worked.

There's ways to intercept SMS messages without sim-jacking or number porting too.

https://arstechnica.com/information-technology/2021/03/16-at...

That is smart, funny and sad, all at the same time.