That very likely means the plugin hasn't loaded at all.
window.open() is a function that already exists before you load the InAppBrowser function. The InAppBrowser plugin replaces it with its own version of it that has extended functionality.
You can test if the plugin has loaded by testing the type of the object returned by window.open(). If it's InAppBrowser, everything's fine. If it's Window, the plugin has not loaded, and you're using the default version of the method.
I followed the steps you laid out in your other post exactly, and when I check the type of the returned object, it's Window. Your solution does not work.
EDIT: Misinformation struck through.