Why does this connection not become a low impedance between Positive (Isolated system) and Ground (Earth)?
its because the V+ in the battery powered circuit is only positive relative to the battery GND. To the rest of the world it's floating/undefined until you connect the oscilloscope gnd. Now V+ of the Battery battery powered circuit is equal to the oscilloscope gnd. Still no current flows. you just have defined voltage potentials now. The Battery battery powered circuit gnd is not equal to oscilloscope gnd but negative (oscilloscope gnd minus V+). So if you now measure at the Battery battery powered circuit voltages will be shown as negative values
So first have a look to the oscilloscope - its galvanic isolated (but neutral and secondary side GND might be coupled). If GND is floating, wherever you connect it, it is tied to the potentials of the device under test.
second is your example in 3 steps starting with no connection to full connection with GND and probe: