The sticker is right. Everything you need is loaded onto internal memory. I believe TomTom puts that sticker there because many people were complaining that they were missing a vital part - the SD card - in order to make their unit work. Not so.
So what is the SD card good for? If you purchase another map, say Western Europe, you can load it, along with the operating system, on to the card. This way, you don't have to disturb your original setup on the internal memory. Just note that SDHC cards don't work on these units.
If the card is inserted, the unit will boot from the card. Remove it and it boots from the internal memory. It's a case of either/or. You don't have access to both memories at the same time.
(Note that this does NOT apply to the x20 and x30 units, which CAN access both. This series can also use SDHC cards.)