Doesn't using webm require sigining away their right to ever sue Google for patent infringement?

No. Originally, the webm license required that you couldn’t sue google for patents related to webm. They switched to BSD in 2013.

Meanwhile, there have been numerous patent suits against google from other big companies around VP8, VP9, and webm. Google isn’t the bad guy in this story.

Thanks I didn't know about those developments!