I've seen references to fairly high current coming from the Batteroo and high temps under short circuit condition.
Comments on boost converters:
Without special circuitry a boost converter is not protected against short circuits. With a basic boost implementation, the boost circuit will have a diode or a PFET (with a parasitic parallel body diode) as the output switch. With a low power, high efficiency boost as the Batteroo, I'd bet it has the PFET implementation. The body of this PFET is often just connected to the output, assuming the output to be higher than the input. That places the parasitic diode to be "pointed" to the output, reversed biased under normal operating conditions. Under short circuit, that assumption is violated and there is a low impedance path from input to output of the ESR of the inductor and the (likely) PFET switch body diode in series. It would be interesting to see if this is the case with the Batteroo.
Also, even if extra circuitry has been added in to protect against the (likely) PFET switch body diode being pointed to the output under conditions where the output is less than the input, there is also the control problem encountered under short circuit. The boost control circuitry will recognize a large error voltage present. In response, it will want to bring ever increasing current through the boost inductor. If the current limiting circuitry isn't designed properly, the expected current limit can be exceeded by quite some amount.
Scope probing of the IC side of the inductor would show whether there is any switching action under short circuits; current probing through the inductor (takes a modification of lifting up one side of the inductor and inserting a current loop) will show the loop response when the output short is asserted.
Don't know if any of this is applicable to the Batteroo implementation, but there are boost design issues which can cause excessive current draw from the battery, not just an insulation short through the sleeve.