The simple answer is - It's a mess.Maybe the law is clearer than my understanding of it, or maybe it is a mess!!
Not one of the four systems is a secure system.
An alarm is an active system but if you can't hear it, it's useless.
A fence or wall is a passive system. It’s easily breached but it doesn’t tell you, so it’s useless.
A rigid cover has to taken off if you want to use the pool. As soon as you take it off there is no security, so it’s useless.
An abri is in effect an indoor pool. If the entrance is breached it doesn’t tell you, so it’s useless.
You pays your money and you takes your pick – of four useless systems.
Alan