The Sega-CD, 32X, Famicom Disk Drive and Turbo/PCE CD-ROM formats are all platforms. They all have their own brand of games, on different media than their base console, which will only play on their own hardware configurations.
You may need to program for the base console to make games for them, but the games still require programming for the add-on hardware.
Outside of "how many systems do you have?" lists, "system" is generally used in video game discussions when refering to a platform.
AC adaptors, RF switches, light guns and controllers are "merely peripherals". A light gun or gimmicky device might have a few games that support it, but stuff like the Sega-CD and 32X are hardware upgrades that create a brand new format that is supported by a real line of software.
Most people seem to make (sometimes personal) distinctions between hardware and mere peripherals. The non-original-console items you'll find on those "systems" lists are likely all generally considered hardware and not peripherals/accessories.
Also, it could be argued that the only real difference between a 32X/CD-ROM combo and a lone Genesis system is that the Genesis was released first. And that it's the 32X/CD-ROM setup that is the real system hardware configuration (as it can play all 3 formats) and that the Genesis is merely a peripheral that enables it.
So whether or not by anyone's definition the people who list platform-base-hardware as systems in lists are right or wrong, it's not hard to see why they would. ;)
