Right, but it took a couple tries to get the courts to understand this. The idea of "software compatibility" was completely novel to copyright judges, and there's no parallel in other creative endeavors. The closest thing I could think of would be writing in someone else's creative universe, but in that case, it's crystal clear you don't get to do that without a license[0]. The courts just decided - later on, when defendants armed with better arguments and copyright hygiene[1] showed up - that software copyright
has to be thinner than other copyrights, else there is no way for the owner of a program to legally separate themselves from the software libraries, ROM code, or OS they run on.
Even then, you don't get to just say "we need this for compatibility", you have to actually find software that breaks if you do it any other way. The act of reimplementation is both reverse engineering technique as well as legal technique - you are building up a series of excuses for specific acts of copying. What Franklin did the first time around was go straight for the conclusion they wanted, which courts really, really don't like. Courts want to see your struggle.
[0] In fact, Oracle's argument against Android in Google v. Oracle hinged on their ability to make reimplementation of Java functions sound like plagiarizing Harry Potter.
[1] The words "clean room reimplementation" get thrown around all the time, but it's not strictly necessary to be clean-room. The precedent for compatible reimplementations includes Sony v. Connectix, where the latter was very much copyright-tainted and won anyway.