EIther way, it can only drive resistive loads since it says it only for heating (duh).
If it has a regulated voltage output, I would say:
You won't destroy the converter by running it less than 500W, but it may not meet performance specs, which means you could destroy low power devices if the voltage is not regulated up to spec.
If it has unregulated voltage output:
then that might mean that it is not able to run if the current is too low. I've seen a few converters that are like that, but this is just a mains step-down, so it's probably just a transformer inside. If this is the case, I see no reason why it wouldn't work.
Best case: it works
Worst-case: it doesn't work due to the converter not being able to supply less than 500W power. your phone won't be destroyed as long as the voltage is stepped down enough for your phone wall-wart regulator to work.