Why can't "unless" mean "except if" in "My wife will be angry unless I'm home by 7.00"

A Practical English Grammar reads:

Unless means ‘except if’, so it’s not used when the meaning is more
like ‘because … not’:

My wife will be angry if I’m not home by 7.00

※ My wife will be angry unless I get home by 7.00
(She will be angry because I’m not home.)

I still can’t see why unless can’t mean "except if" here.