Yes I have confirmed from them. However this is applicable only if the pre-deductible payment (through a corporate plan for instance) is more than the deductible amount.HDFC doesn't says this in policy wordings, and if it is a hidden clause it is very wrong.
And yes, once you opt in for deductible, you can not opt out before 5 years.
For instance if the hospital bill is 5 lakh and, corporate coverage is 3 lakh and HDFC policy deductible 1 lakh, corporate will pay the first 3 lakh and HDFC will then pay the remaining 2 lakh on a reimbursement basis.