Buyer accepts offer and signs it, where offer acceptance is binding depends of the legal jurisdiction. For example English and common law differs slightly from American uniform commercial law on matters of offer acceptance and when performance becomes binding on an offer. The seller can also reply with a pro forma invoice. In fact a pro forma invoice can serve as quote and offer, becoming a more or less binding purchase order upon buyer acceptance and signing. Pro formas are used more in smaller FCL trades, and not in larger bulk trades.
Even taken at their word, it doesn't change the fact that they are using documentary procedures that are unusual, and abnormal. Contact any business college, or speak to established traders or people working for large multinational corporations. Talk directly to international bankers on this.