Galvanic isolation means that no (significant) current can flow between two parts of the circuit (here, the input side and the output side). It doesn't mean the output voltage of a circuit has to be lower than the input voltage.
Galvanic isolation is very important in mains power supplies, because a failure to galvanically isolate the two sides may put the low-voltage output side of the power supply (and whatever it's powering) at mains potential.
This galvanic isolation is the subject of safety regulations, and can be a real concern with low-quality power supplies. See for example this teardown of a fake Apple charger, or this teardown of a USB charger (that is actually putting mains on the USB ports!)
Galvanic isolation in step-up converters
In a step-up converter (and indeed, anywhere else), galvanic isolation still means the same thing: no (significant) current can flow between two parts of a circuit.
One way this can be relevant is if the input side of your circuit is (or can become) earth-referenced. With galvanic isolation, there is only a voltage across the two output terminals, but the output terminals are floating with respect to earth ground. A shock risk should only exist when someone touches both terminals.
However, if the galvanic isolation fails, one of the output terminals may become earth-referenced, putting the other terminal at a high voltage relative to earth ground, creating a shock risk when just the one terminal is touched!