Is the prefix "pre-" meaningless in the terms "pre-heated" and "pre-board"?

This question poses a paradox of meaning. The general question is whether, if two sentences (x and y) can be used in the same situation, with the same literal meaning, and x and y only differ in that x has morphological affixes that y doesn't, aren't the surplus affixes "meaningless" by the alternation test?

Take this example:

I put the chicken inside a preheated oven.

Wouldn't it make more sense to say:

I put the chicken inside a heated oven.

There is only two states an oven can possibly be in—heated or unheated.

Another example: Does first-class board the plane, or pre-board the plane? What does that mean—to get on before you get on?

In these cases pre- means "earlier than usual," or "ahead of time". Saying "We will now commence pre-boarding for our gold medallion members" means the same thing as "We will now commence boarding for our gold medallion members," except that it also carries the additional implied meaning: "Gold medallion members are entitled to board earlier than others." Of course, it wouldn't do for the airline to say "Gold medallion members are entitled to privileges that the rest of you aren't. Therefore, we will now commence boarding for our gold medallion members. The rest please wait even longer." The pre- lets them communicate the same thing without actually saying it.

So to answer your question more generally, if you can say two different things in the same situation, it doesn't mean that they mean the same thing.