I can't help but feeling like if we're just going to be talking about copyright, we've jumped the shark.

I said it's informative, not entirely on point, which is effectively what you're expanding upon. In legal parlance that means some of its legal principle is applicable, not all of it. "On all points" means the legal principles have direct bearing on a case in substantially all details. "Not entirely on point" means there's a point or two that are influential, but has no direct bearing as in an all points precedence.

The applicable parts are those files that could hold copyright in an otherwise mixed and very muddled environment of pre-Copyright Act files (pre Copyright Act of 1976*) and those with muddled authorship that couldn't be clearly established but substantially or wholey written after 1976. The few files that were clearly copyright AT&T were removed and replaced in the BSD with clean replacements after the 1994 settlement. To imply that all of Unix was under trade secret rather than copyright law is not true since Unix (both AT&T derived and Berkeley derived) clearly evolved after 1976. For example, Unix System V was released in 1983 and would very clearly have been largely copyrightable if provenance had remained undiluted. The details of a case matter. Some matter more than others.

*Apple v. Franklin (1983) ruled machine code, and by extension human readable code, are in fact covered under the Copyright Act. Google v. Oracle (2021) affirmed header files are also covered by the Copyright Act. That means all source code, functional and descriptive, since 1976 is potentially covered by federal copyright. Not that there weren't copyright laws before 1976, the states managed that with contradictory legal regimes that may or may not have covered human readable source code. It was the complexity, contradictions, and inconsistency of state copyright regimes that prompted the 1976 Act.