"But nothing in there clearly states what constitutes threatening, abusive or insulting behaviour."
No it doesn't, nailit. That's what courts are for. If a defendant says he did not commit threatening, abusive or insulting behaviour and the alleged victim believes that he did, then a court must decide who is right.
There are many "vague" laws. One that may be easier to understand is careless driving. The law says that a driver is guilty of careless driving "...if (and only if) the way he drives falls below what would be expected of a competent and careful driver." It is for a court to decide (assuming the defendant pleads Not Guilty) whether his driving fell below the expected standard.
In fact the difference between that and dangerous driving (a far more serious offence which attracts a prison sentence of up to two years) is simply one word - "far": "...a person is to be regarded as driving dangerously if and ....only if the way he drives falls far below what would be expected of a competent and careful driver" There is a little bit of qualification which I have left out but the essential elements of the offence are as I described. It is for a court r=to decide if either of the offences have been committed and they do so based on the evidence they hear.