Well, let's see.
1. Officially licensed tie-in material? Check!
2. Approved by Gene Roddenberry? Check!!
3. Used on screen in TMP? Check!!!
Without even leveraging the word "authentic", these three points make a strong case that they are
bona fide.
I've already characterized FJ's work as establishing a benchmark.
To elaborate: they depict a version of the imaginary starship, a version that because of the three points above has legitimate standing. There are numerous ways to resolve inconsistency in-universe. It's already been pointed out that perhaps the plans are specifically of the USS
Constitution. Minor variations in ships of the same class are a well-known thing, IRL. Perhaps they represent the plans of an intended refit post-series. Looking at the Forewords in the tech manual, other alternatives present themselves, such as that perhaps the discrepancies are the result of redactions in the timeline by Starfleet.