I think I'm just confused, but Section 5.3 states, quote:

The transition from "Data Sent" to "Data Recvd" happens immediately if the application resets a stream and all bytes up to the specified Reliable Size have already been sent and acknowledged. Conversely, the transition might take multiple network roundtrips or require additional flow control credit issued by the receiver.
(snip)
Similarly to the sending side, transition from "Size Known" to "Data Recvd" might happen immediately or involve issuance of additional flow control credit.

I wonder if the text is incorrect to point out that additional flow control credits might be required.

Reliable Size is required to be no greater than Final Size. Therefore, the moment the sender sends a RESET_STREAM_AT frame, it should have already received and consumed the flow control credits issued by the receiver.

I think the fix would be drop the first reference and change the second one to talk about packet drops and retransmissions.