Actually, I said if he is going to use s-video. I'm not that familiar with SCART and the various connection options available to UK X1 owners, but in this case he stated that he has s-video output so I suggested if he was going to do s-video then he has no need to buy the adapter.
You are correct that interlaced component input via the s-video adapter cable utilizes the DCDi, but so also does the s-video input (and the composite interlaced input). So DCDi is used in either case.
In theory, interlaced component should be better than plain s-video, but in practice many X1 owners have said they see little or no improvement over s-video. Many users have reported much more noticable improvements by going to progressive component, but even that seems to vary depending on the user's equipment.