That is, of course, assuming there are still IP issues. I'm not sure what made them concerned about that, if anything tangible.
Notably, the Opus license is voided when you enter Opus-related patent litigation with an Opus user. I think many(most?) companies which hold patents relevant to Opus also license Opus for their products; so they would be trading patent litigation for copyright litigation.
From the Opus licensing page¹:
When it comes to patents, it is difficult to say much without making lawyers nervous. However, we can say something quite direct: external counsel Dergosits & Noah has advised us that Opus can be implemented without the need to license the patents disclosed by Qualcomm, Huawei, France Telecom, or Ericsson. We can also say that Mozilla is confident enough in Opus to ship it to hundreds of millions of Firefox users. Similarly, Cisco and Google are also supporting Opus in some products. More companies are expected to do the same soon.
Mozilla invested significant legal resources into avoiding known patent thickets when designing Opus. Whenever possible, we used processes and methods that have been long known in the field and which are considered patent-free. In addition, we filed numerous patent applications on the new things we invented to help defend the Opus community. As a result, Opus is available on a royalty-free basis and can be deployed by anyone, including other open-source projects. Everyone knows this is an incredibly challenging legal environment to operate in, but we think we’ve succeeded.