That is historically how you formally address former presidents.

It's a weird thing they do for anybody that held an official title previously (maybe just in America), I've seen it before for Mayors, Governors, Senators, Presidents. It seems like such a confusing thing, like shouldn't they be referred to as "Citizen [[name]]" or something, to indicate the change in position?

Being willing to performatively honor Trump's delusions, e.g. calling him the president because he is happier with that than with being the former president, is a key requirement for him to be happy with your job performance. It is probably the most valuable requirement for working with him, in his mind.

Because God-king has yet to be installed as an official title

I believe in this case not including "former" makes more sense due to it being lawyers speaking on behalf of former President Trump. He may be named as such in court documents as well.

If it was you and I speaking of former President Trump we would use "former". If you were speaking to former President Trump, you would address him directly as President Trump unless you were intending to be "disrespectful".

Used to be normal to call a former POTUS "President".

https://uk.usembassy.gov/our-relationship/contacting-former-presidents/

OTOH, I'm reading that's not correct. 🤷🏻‍♂️