DZGUI gets its server information directly from the most authoritative source: Steam. It does this by letting the user be solely in control of their own API key and the application in an authenticated way. Users explicitly get permission to use a Web API key instead of scraping DayZ server info from third-party sites.
Everything that happens between DZGUI and the Steam Web API endpoint takes place solely on the user's computer, using a GET request (fetch server list), and no information gets sent back to the developer. DZGUI does not scrape third party DayZ APIs without permission.
There is some misconception that a Steam Web API key could be used to gain information about a user's account or control their account. Not only is this not possible, but the Web API key is used solely by the user on their own computer and is protected by Steam Guard.