Markups Core enterEditMode() returning false

Question

function newMarkupGUI(viewer, options) {

  Autodesk.Viewing.Extension.call(this, viewer, options);
  thisViewerId = options.id;
  this.viewer.loadExtension("Autodesk.Viewing.MarkupsCore").then(() => {
    let extension = this.viewer.getExtension("Autodesk.Viewing.MarkupsCore");
    extension.enterEditMode();
    console.log(extension.enterEditMode());
  });
  
}

When I am inside my main js file where I initialize the viewer, I am able to access functions such as enterEditMode() like so:

var extension = viewer.getExtension("Autodesk.Viewing.MarkupsCore");

extension.enterEditMode();

This works. But inside my extension called newMarkupsGUI, it seems getExtension() does not work. I am confused about how this all works, as the documentation is pretty sparse. I would rather keep my extension separate and not hard code the functionality of markups where I am initializing the viewer. Any help would be appreciated, thank you.