"Period Correct" is a sliding scale. As in the case of, say Hot Rod Cars, you are talking about an industry of modification spanning nearly 75 years. The term 'period' for hot rods, as in, "That would be a period correct modification for that build' would mean that the part would be sympathetic to the build and the period, but not a specific year. It is easier to talk in terms of period mods because the periods are seen as much broader.
In mountain bikes, the history of the modern mountain bike is less than half the age of Hot Rods, so it stands to reason that the 'periods' are much more defined. That does not mean, and I do not think, that they are year specific however. There are certain defining moments in mountain bikes which can be viewed as period specific. Suspension forks, 80mm forks, 100mm forks, v brakes, disc brakes, STI, thumbshifters, Suntour MD, CNC, Fad colors, etc. Designating each of these trends to a specific year and trying to narrow the term 'period' to that level is a mistake. Period means the parts are complementary to one another and to the frame. If you went back in time to 1994 with your bike, would anyone say "Holy shit! Lookit that new fangled thing!" or, "Damn, son...get rid of that creaky POS and upgrade to XYZ" if yes, then you aren't period.
Specific year means nothing to the term "period correct" in cars or bikes. In cars, "Correct for the Year" is the term that I think Dr S and others are defining. I'm not trying to split hairs, and I'm sure there are international regionalisms that apply, but again...in my experience both in cars and in bicycles, "Period Correct" and "Correct for the Year" are very different things.
Reproduction parts, for example, can never be "correct for the year". That would make them original or NOS OEM stock or salvaged. Repro parts are "period correct".
Dr S is actually right and wrong, I think. His retro Schwinn is "period correct" because it is sympathetic to a phase in cycling history. The parts, though, span 75 years. W're understanding of that being defined as period correct, but when the bike gets a little newer and the "periods" become smaller chunks of time, then folks want to act like "Period Correct" is year specific? Hogwash! Why wouldn't a build up of a 1992 Fat chance be considered the same "period" style without meaning that every nut and screw and component came was produced in 1992. If somebody hunted and pecked to make sure that all the parts were either new in 1992, or at least available in the same rendition in 1992, then that build would be "Correct for the Year"
Clear as mud.