Search on the App Store is a bit of joke at the moment so text in the app description is more or less useless; any possible search terms really need to be included in the list of keywords if they're going to be at all effective. Stack Overflow and the rest of the trilogy sites are included in Stack Up's keyword string which is why it's showing up in search results.
Developers need to submit a new binary if you want to change the list of keywords, so I'd reach out the other developers and get them to add the trilogy and "Stack Exchange" when they submit their next version.
For what it's worth, here's the keywords string I submitted with Stack Up:
Stack Exchange, Stack Overflow, Server Fault, Super User, Questions, Answers, Programming