It's all about exposure. They'll get the chance to play in front of a very different crowd at Slam Dunk and whatever stage they play will be packed out for an early set because so many All Time Low fans will be there and willing to watch anything Alex does. Lot of people might pop along just because it's Mark Hoppus too.
A band generally wouldn't bother opening a stage at Slam Dunk if they were headlining the 3rd stage at Download 2 weeks later, you're right, but at this point in time Simple Creatures will want to get their music and EP in front of as many people as possible. Playing both Download and Slam Dunk is a big way to do that in the UK. No idea if they're planning any EU shows or fests as well, but as well for Hoppus to come over to the UK other than doing some press for Simple Creatures/Blink, makes a lot of sense that they'd do a show at Slam Dunk too.
They won't need to protect the value of the brand/band as there isn't one at the moment anyway, they're not in a position to not want to open a stage at a festival or cry about their logo being smaller than another band that's bigger than them anyway (OH HI ARCHITECTS). It being billed as a special guest appearance protects their brand anyway as it's a special thing, not just them opening the stage as if they're a small nothing band.
It makes perfect sense that they'd do it and I'd be surprised if it wasn't them at this point.