I don't keep my tongue pressed firmly against the roof of my mouth, nor have I heard of others suggesting this either. If this were something all humans were supposed to do, it would probably be more commonly known. Further, how you hold your tongue is highly unlikely to have any effect on how your head, neck, or posture develop. I am not sure from where you got these ideas, but I do question their correctness. I have not done any of this and my head is not poking forward to an unusual degree.
While perhaps this is pedantic, the Starting Strength model is not perfect. Nothing is. As detailed as the advice in the book is, I don't think Rip specifies what you should do with your tongue. Getting stronger will generally not have an effect on bony changes to skeletal joints. It may help you stand up straighter if you were slouching because you did not understand how to stand up. It will not correct scoliosis.
This may just be you at this point. People have skeletal abnormalities, although there is a wide range of what is considered "normal." If you cannot breathe properly when you put your head back, then I would prioritize breathing over any ideas of what constitutes ideal posture. If your cervical spine has developed in this way and you are an adult, it may be difficult, or impossible to change. If you are not in pain and you can go about your life in a full and complete manner, I would not worry overmuch about this. I strongly suspect this had nothing to do with your tongue, or how you were breathing.