If you pay with your CC in a store, you still say the store accepts the CC. Even though they get fiat from the CC company.
The credit card itself is based on fiat.
Overstock does not accept BTC for a later use to...restock itself using BTC to pay it's suppliers. Etc... That would be an actual acceptance of BTC.
What they offer is a way for you to pay in BTC but what they accept is fiat.