I am not sure where people are finding their attorneys. You're one of several posts where I saw an attorney giving wrong advice. When hiring an attorney, make sure they are an immigration attorney. A family law attorney may be good at creating prenups or filing for a divorce, but they are not an expert on immigration. I was an attorney for 15+ years but I practiced in the area of wills, trusts and estates. I would never attempt to file an immigration document for a client. That's like asking a dentist to perform heart surgery. Attorneys are well versed in their area of practice but when they try to engage in another practice area, expect lots of mistakes. With that said, I married an immigrant back in 2018 and I hired an attorney for that process. The attorney was well known in immigration law and did an amazing job for us. Unfortunately, we divorced but I met someone new and I am going through this process again. Again, I got an attorney who is extremely knowledgeable in the area of immigration law. Could I do it myself, sure. but why not have an expert do it for me?
Back to your question, yes a letter of intent to marry is a requirement of the I-129F form. Your attorney should have known this as it is basic information. Unfortunately, failure to provide it will either cause your file to be denied or RFE. Good luck.