Thats not true, I don't know the technical limit but most stations I get usually have 6 or 12 hours worth and some have 24 hours worth.
My OTA STB's guide will advance out a week but its never that full
You are right. After I made my post, I did a little research. I found this:
The ATSC PSIP standard (A/65) for Digital Television allows broadcasters to identify themselves and convey up to 16 days of programming information. And this very large PDF about PSIP.
I guess the critical words are "up to." I haven't looked at my OTA stations PSIP in a while, but when I last looked, they only did the current and maybe the following program. I'll have to check again.