Earlier quoted context omitted.
If the BIOS API was copyrightable, they would have had to do nothing and simply declare that they owned the copyright to the API and thus no clones. Since they couldn't do that, they had to go to the extra step.
Replying to your comment below. I am not missing your point. I am merely pointing out that your point is incorrect as a matter of law. Copyright does not cover re-implementation. Copyright only covers copying. Your statement that IBM could sue company using a clean-room developed re-implementation of IMB's BIOS if IBM owned a copyright to their BIOS is incorrect as a matter of law.
Clean-room reverse engineering produces an implementation that is not a derivative of the original implementation. But if the API itself is copyrightable as a separate work from the implementation, then you have to consider whether the API of the reverse engineered implementation is a derivative of the API of the original implementation. The answer to this question is rather obviously yes, since the reverse engineering process is all about separating the API from the implementation and copying only the former. Clean-room reverse engineering is only useful in a world where APIs aren't copyrightable.