Yes, and the amount will be booked right back if the chargeback is eventually lost by the issuer/won by the merchant acquirer. If that happens, it's up to the issuer to further claw it back from the cardholder (if permitted by rules and regulations) or write off the loss.
> There is zero chance American Express just picked up the six figures of chargebacks for the folks who put them in today.
Receiving an immediate credit is quite typical (and in some cases required by regulations), but receiving the money back is by no means an indication of having won the chargeback case. The keyword here is "provisional credit":
> When a chargeback is filed, the process to reach a final verdict can extend for weeks or even months. During this waiting period, issuing banks typically provide cardholders with a provisional credit covering the amount in question. If the resolution favors the merchant, this provisional credit can be easily reversed. (from [1])
All of this is usually also mentioned in the cardholder agreement and the "boilerplate" text you receive as part of a provisional credit from the issuing bank. Definitely both worth a read, especially when large amounts are involved.