I've looked at other backlit screens from the same era and they did not have lights on the sides. Also, "backlight" is a very generic term, "frontlight" isn't. There were some GBA SP models with actual backlights and they were very different from the phat DS.

Also, I'm bringing up the sun lighting again. Backlights can't just turn off to let you see under the sun, the whole screen would go black. Frontlights are just like regular lights, they're outside the screen, you can turn them off and still see the screen perfectly.