Earlier quoted context omitted.
We ran the Opus license terms by the FSF before publishing them, and they confirmed that the terms were compatible with the GPL (v3 or otherwise).
Wasn't there an issue with a couple of companies filing IPR's with royalties that cover parts of Opus?
Please see the "Other disclosures" section on https://www.opus-codec.org/license/
You may also be interested in https://hacks.mozilla.org/2013/02/defending-opus/ (warning: contains discussion of specific, unexpired patents).