Yes, you are missing something!
SCART can carry either
RGB video (with the synchronization signal on the pin that would otherwise be used for composite video, it uses a total of 5 pins),
S-Video (using one of the pins used for RGB signals and the pin for composite video, it uses a total of 3 pins),
composite video (it uses 2 pins).
SCART can only carry one
of these signals at any one time, and only if the sending device supports that signal. Of course, the receiving device must also support that signal
Some devices can switch which type of signal they can send. In Europe, DVD players for example often can produce RGB or S-Video or composite video. Often the selection is done via an on screen setting menu screen. But some devices can only produce ONE
of these video signals. For example, the VCRs in Europe often only had a composite video output available on their SCART sockets.
The adaptors that you have linked to are for use when a SCART socket already has
a composite video output available and you want to connect it to a Phono/RCA connector/lead. They cannot convert RGB to composite video.
While taking about analogue video, I should point out that there are big differences in quality when colour video pictures are involved. The best quality pictures are RGB video, the next best is S-video, composite video produces poor colour pictures and the worst is RF/VHF/UHF video. So where possible, the preferred option is RGB video.
More on SCART here