How does this isolation transformer protect user from transients/mains?
Mains transformers protect in two ways:


By providing isolation
By limiting energy with a saturable core


Transformers usually have a core made of a material that has a saturation point (like ferrite), this limits the magnetic energy that can be transfered from one coil to another. This is nice because if there is a short on the secondary, it won't get the full power of AC mains. The transformer should have a power rating, like 24VA, this lets you know how much power can bet transferred before saturation (or heating). 


Source: https://www.quora.com/What-is-the-core-saturation-of-a-current-transformer

The second thing a transformer does is provide isolation, which means no DC current through the core. This is nice for saftey reasons because only AC currents can get through the core, spurious or unwanted DC signals are blocked. It also means that the secondary side can be grounded to the chassis, in the event of a fault the current flows into the chassis and out to ground instead of creating a potential.