I’m not sure I agree.

Here’s my interpretation of what you mean by semantic associativity: “Bi-” is another way of saying “two,” and the confusion with “biweekly” arises because it’s unclear if the “two” should be taken to mean “two occurrences per week” or “two weeks per occurrence.”

Meanwhile, “semi-” means “one half,” and in normal conversation it’s not common to say that an event occurs “one half of a time per week”. So it’s unambiguous that it must refer to “one occurrence per half week.”