Frustratingly enough, it’s the other way around. Biweekly is every two weeks, semi weekly is two times a week.

I remember it like this:

• bicycles are two circles, biweekly is two weeks
• semicircles are half a circle, semiweekly is half a week

But yes, people use the words interchangeably so often that it’s faster just to avoid the problem altogether and just say “every two weeks”.

Probably because it’s off topic for the community