I thought the same, so I went to the dictionaries. According to the Oxford English Dictionary, “biannual” can mean either twice in a year or once every two years. This was news to me, since I had learned that “biannual” (or preferably “biennial”) only meant once every two years. Merriam Webster’s Collegiate Dictionary defined “biannual” as twice a year, but defined “biennial” as once every two years. Confusing, no?
I agree that “semiannual” should be used when the writer means twice a year.