As the Learn More link on the screen entitled Tell us about anything received in medical reimbursements says:
"Enter reimbursements for medical expenses already paid.
Do not enter an amount if you:
- Entered only your co-payments, or
- Entered amounts you paid to satisfy a deductible required by your insurance company, or
- Subtracted the reimbursement from the amount you paid when you entered it"
So if you previously reported only net medical expenses (i.e., after insurance reimbursement), enter nothing here.
But, yes, you could have entered the original amounts paid and then entered the insurance reimbursement, and TurboTax would have automatically netted them.