This is an acceptable way to equally share the reverse voltage in a string so that -15V may be shared equally to -5x3 when AC is applied within this range of <= 15Vp
Or multiples of this in larger voltages.
How did they select this value?
Red LEDs are often rated at 1uA leakage and White 10uA due to the dielectric with reverse voltage -5V max. Thus this is equivalent to 500Kohms @ -5V. In order to prevent unequal series voltages from over-stressing one component, leaky resistors split the voltage more equally when sufficiently lower than the component leakage.
So by being conservative on peak reverse voltage and thus slightly higher effective leakage resistance, 300 k will dominate the voltage drop. Without specs, not much more can be specified.
The same method is often used for electrolytic caps to share Vdc equally as both the LEDs in reverse and e-caps in forward are in "insulation mode". In the opposite polarity they are both in conduction mode, although unhealthy for e-caps and luminous for LEDs.
A better way is to include reverse diodes in the chip done on only the best components.