Not true at all. I've done this for years. If they don't accept my reasonable changes, I get up and walk.The big long form with the total being loaned, APR, term, total amount to repay is a standard loan contract most lenders will accept. They will not take one with edits.
Sometimes I get what I am asking for, sometimes I don't. Doesn't matter as it not life or death. If you have never done this, then don't just assume they won't accept it - because most of the time, they will. Making a sale matters when you have numbers to meet.
Like I said, I've done this for over 30 years. Learned this in a financial management class from my professor. Others scoffed - I listened.