Relevance of Galvanic Isolation of Boost Converters
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!