I am Dr Mbong. Your insurer probably refers to an internal disease as one which is hereditary while external ones are acquired (the opposite of hereditary).
can be due to hereditary (internal) family disorders like sugar disorders like galactosemia
etc. They can also be caused by acquired (external) diseases especially infections got from mother to child like congenital CMV, Rubella
So I dont think the worry of the insurers is if it was diagnosed at birth or later but if it is a family thing or secondary to some disease whe got while you were still pregnanct with her.
'Internal' or 'external' causes of congenital catarcts can be diagnosed immediately at birth or develop later (and therefore diagnosed later). Its internal or external nature does not assure when it will be diagnosed.