I happened to see this question
To my knowledge , a developer of a theme or site must declare if its theme is responsive or not.
If responsive ,the presentation on the mobile device is adapted responsively by the site CSS , to keep it readable for a user of the mobile device with a small screen.
Important for the layout presented on the device is the number of pixels the device can handle .A responsive theme with CSS reacts to device capacity of pixels and adapts the layout to keep things readable. Thats the trick. If it can not be done, it is not a responsive web design.
All this responsive efforts are made for the benefit of mobile users to keep it readable in a different layout for mobile.
So this question has nothing to do with the theme ,which is declared and functioning responsively ,but with the manufacturer of the mobile, who offers the option to choose between a fixed layout called desktop mode and a responsive one adapted to the device.
Glad I can give answer to the question , to my knowledge and contribute
Danny