I noticed that as well. But there are two important points here.
1. This is the Sun. Highbrow for them is wearing a tie. Research is whatever can be picked up in the local pub.
2. The fire brigade is managed on military type dicipline. The senior officers are given the same respect as senior officer in any military outfit simply because those under them must obey commands instantly.
It seems highly unlikely that any such directive has been issued. What is more likely, is that some reporter met with a fireman in a pub and asked what would happen if they called the comander, sir or referred to then as he.
The answer is, they would get the same response as they would if they used an inappropriate adress to any other senior officer.
We can imagine if someone in the US miliraty adressed the President as madam.