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.

UNLESS: https://oed.com/oed2/00268144

There is no significant difference in meaning between the two, so I wouldn't worry over it. I'm not sure what the "X" is on the second sentence, but both are grammatically correct and commonly used.

If there is a difference in meaning, it's very subtle. There's a similar question on another Stack Exchange and the general consensus is this:

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

This implies that my wife is not angry. However, if I get home after 7:00, she will become angry. Arriving after 7:00 will trigger my wife's anger.

My wife will be angry unless I get home by 7.00.

Here, "unless" means "except if". The sentence implies that my wife will be angry even if nothing else happens. But, if I'm home by 7:00, I might be able to prevent her anger.

The difference is so subtle that it would almost never be noticed during a spoken conversation.