As far as I know Bubble detects the browser’s user agent – it’s not looking at the devices screen width at all.
Some emulators won’t work to test, because they aren’t actually changing the user agent but are just showing a small browser window.
With most browsers, changing the user agent is pretty easy. 
If you own a Mac, installing the Xcode (free) Simulator makes testing on virtual iPads/iPhones even easier/better. (Just download Xcode from the Mac App Store.)