That is the license by the creators but does not mean other companies/individuals won’t claim that it infringes on their patents. So if you adopt it, you’re still likely to be accused of infringing their patents.
and it's backed by these companies: https://aomedia.org/membership/members/
https://www.sisvel.com/blog/audio-video-coding-decoding/sisv...
It became a real legal issue before the first consumer AV1 hardware was ever launched.