Well, you can assign a meaning to lowest/highest on a single 2-9 too.
And no, it wouldn't really indicate a rocket by default if you don't have a convention. It depends on the circumstances.
If you are tasked to take the 7red and have only the 1red, the 2 ways to communicate(3 ways in fact, because saying that 1 is your lowest also clearly can be conventional in that scenario) would either indicate that you can to win the task by playing a rocket on a red trick or that something else must be found (you'll have to do tricks in some other color and have someone discard the 7 red). This can for instance save time if there are other tasks to be done afterward.
Now, whether such conventions are in the spirit of the game or not, that is up to every game group to decide for themselves.