NJROTC will teach some naval history (in one of four total textbooks, the others cover basic maritime concepts, citizenship, and leadership), but it is almost exclusively naval history with relatively little else. There is significant focus on maritime battles, at the cost of leaving out pretty significant chunks elsewhere--not American history, but American *naval* history.
It is very much oversimplified (early civilization to the modern era in about 150 pages) and is definitely not anywhere close in detail to the level of any AP or honors history course. The textbooks are standard nationwide (I believe published by the Naval Institute Press) and must be accessible to all levels of students. The purpose of the program is not academics (many schools use the JROTC program as an alternative to P.E.), and the textbook may not even be covered it its entirety, as sparse as it already is (e.g. maybe 1-2 lecture hours for academic material a week, the rest is leadership, seamanship, drill and ceremonies, uniform inspections, etc.).
The oversimplifications pointed out in above NYT article are probably accurate. The classes are also taught by military retirees who often don't actually have teaching credentials. Whether any particular pair of NJROTC instructors is able to teach history accurately while faithfully identifying every point of improper bias (personal or in the course material) is questionable. I think most of them just teach the curriculum as-is, per prepared lesson plans issued by NETC.
But to be fair I think the same could be said of many other high school teachers. College professors are definitely even worse when it comes to bias and opinion. And those people have teaching credentials and Ph.Ds.