I also had a problem with this. I ended up just making my own function for it using getBoundingRect()
import wixWindow from 'wix-window';
export function formFactor() {
return wixWindow.getBoundingRect()
.then((windowSizeInfo) => {
if (windowSizeInfo.window.width < 751) {
return "Mobile"
}
else
if (windowSizeInfo.window.width < 1001) {
return "Tablet"
}
else
return "Desktop"
})
}