The term "RCA" refers to the physical connector on the equipment you are using, not the video signal type:
RCA connector - Wikipedia
"Component" video uses 3x RCA connectors - red, blue and green.
"Composite" video uses a single RCA connector - yellow.
A red & white pair of RCA connectors run in parallel with the Component/Composite video feeds to transfer stereo audio.
As Composite and Component video signals are different, the converter listed in the eBay ad won't work. I don't know if the original xbox supports component out, but if it does you could purchase a component cable for it and still use the converter. Alternatively, you would need to get a specific Composite to HDMI converter.
Also, the term "RGB" refers to yet another type of video signal - typically transferred over SCART. The converter doesn't handle RGB signals, so there's no reason why the eBay listing should have that in the title. I'm guessing this stems from the fact that the device uses Red/Green/Blue cables, but it just confuses things for people who aren't so well clued-up on this equipment.