New construction, mortgage questions

US, NC

We signed the contract on a new house a few weeks ago. The contract was just signed (ratified, maybe?) by the builder this week. By which I mean to say the lot is just an empty lot still.

Our realtor recommended a mortgage broker he knows who can apparently get the same deal as the builder's preferred lender (some amount of closing costs). Which, okay, that's fine in theory, and we got a pre-qual letter from them (among others).

But today the mortgage broker emailed us to ask if we were ready to sign the loan disclosures now, and if we would prefer to do it electronically or in their office.

1. They literally haven't even broken ground on the house yet.

2. No explanation has been provided of what a loan disclosure is or why we should be signing it when we wouldn't close on any potential mortgage for months.

I get that mortgages are complicated and people are dumb and that I can just google it, but I've noticed all the brokers I've dealt with so far (including a larger bank, PNC) are kind of shady, doing stuff like conveniently leaving out mortgage insurance on a loan estimate that would obviously require mortgage insurance. I know I'm at an information disadvantage, and that right now they can do whatever they want with all the numbers in the boxes. I'd planned on getting myself better informed by the time we had a closing date.

In short ... what are they trying to get me to sign, and is there any reason for me to do it months ahead of closing, on a house that doesn't even exist in county permits yet?

Thanks in advance.