The letter is correct.
Student Finance England (SFE) make three payments to the OU, 25%, 25% and 50%. The payments are made long after the module starts.
The OU do not need to see the letter. By giving the OU your CRN (customer reference number) from SFE you are registering on your modules, as opposed to simply reserving. By providing the CRN to the OU you are telling them that SFE are paying for the module for you.
Therefore that letter you have is your confirmation that you have been approved for the student loan. So you are now safe in the knowledge that what you have told the OU is going to happen, that SFE will be paying for you.
Don't worry that the payments are made after the module starts. The OU will liaise with student loans regarding receiving payments, this will happen behind the scenes.
Hope this helps, let me know if you have any questions.