Hi Altruin,
Yes you are right, if you console log the wixWindow.formFactor it will return desktop for both cases ( mobile - desktop ) in the preview mode. Regarding the way to test the code if it’s working properly, you can publish your website then take the URL to test it on your mobile, Or if you only want to hide some elements in the mobile version, you can hide elements without coding, just click on the element in the mobile editor and click hide element (eye icon ).
Here is a good documentation about displaying elements on mobile only : Velo Tutorial: Displaying Elements in Mobile Only | Help Center | Wix.com
Hope this helps you.
Best,
Mustafa