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.
LOI is fraught with many legal booby traps & pitfalls especially for the buyer. But probably the most damning reason why credible crude buyers would have little or no use for LOI in their buying dealings, is that using the LOI is fraught with many incalculable legal traps and pitfalls much of which could atimes be very costly for, and to the detriment of, the buyer, according to legal authorities and contract law experts.