I’m unable to reproduce this on Windows 10 using the latest Adobe Audition (build 188.8.131.52). I’ve tried building a default JUCE audio effect VST3 against both master and the latest tip of develop and both display the GUI correctly when opening in Audition. Are you using the latest version of Audition? Perhaps it has been fixed in a recent update.
That isn’t what that line is doing - the check for Audition or Premiere is there to ensure that an editor is always created in those hosts since there is a ref-counting bug which was previously causing editors to not be displayed. See this thread for more info: