The ISO is strictly Software. Has nothing to do with firmware.
The software is the same. Windows 10.
Technically not true ....
For instance the 11/2 Firmware update contained...
November 2015 updates
The following update will be listed as “System Firmware Update – 11/2/2015” when you view your update history.
Note
When Surface updates are provided via the Windows Update service, they are delivered in stages to Surface customers. As a result, not every Surface will receive the update at the same time, but the update will be delivered to all devices. If you have not received the update then please manually check Windows Update later.
HD Graphics 520 driver update (v20.19.15.4308) addresses some display issues, including some scenarios that result in screen flickering.
Display audio driver update (v8.20.0.659) supports compatibility with the updated graphics driver.
Surface Embedded Controller Firmware update (v103.855.257.0) improves system stability.
Surface Pen Settings driver update (v10.0.302.0) adds support for future functionality on Windows 10.
A future ISO would roll these into the ISO as it has both the .wim and the drivers rolled into it to support the surface 4 and a plethora of other makes.