Changing the stroke width for a geojson data source does not render it any differently. It’s always 1 pixel wide (even though it says the default is 2). Even looking at the sandcastle example here…
…if you change the width to something else, run it, and then switch to the Basic Styling tab, it doesn’t reflect the changed value. I’ve tried 3, 30, 300, 3000. They all render identically.