He said that is flat mate said they couldn't use them after 10pm. I
replied they can use them any time they like. How is that not
consistent?


He didn't say that. Read it again. He said "shouldn't", not "couldn't".

That's a significant difference.


No, thats being pedantic.

In any case, I have never been aware of any law, instruction,
recommendation, guideline, or anything else to say that sirens cannot
or should not or any other wording you would like to use, be used after
any particular time.