Can upper motor neuron lesions cause hypotonia?
The rule-of-thumb you (and I) were taught reflects the role of the central and peripheral nervous systems in establishing muscle tone. Deprived of CNS regulation, alpha motor neurons increase in their responsiveness to spindle afferents, resulting in hypertonia. Deprived of PNS regulation, muscle spindles are less able to react to input, resulting in hypotonia.

Beyond this rule-of-thumb, things aren't as black-and-white. As you have pointed out, there is such a condition as central hypotonia. In fact, the NINDS includes the CNS as an area where damage can cause hypotonia:


  "Hypotonia can happen from damage to the brain, spinal cord, nerves,
  or muscles." (source)


Purves et al. also note that upper motor neuron syndrome involves an "initial period of 'hypotonia' after upper motor neuron injury" (Neuroscience, 2012, p. 395). 

To your question: Yes, the rule-of-thumb is not always accurate and upper motor neuron lesions can cause hypotonia.

If you're looking for a hypothetical mechanism, consider this: Purves et al. also state that hypertonia is "probably caused by the removal of suppressive influences exerted by the cortex" (p. 396). So it is conceivable that overstimulation of these inhibitory control systems by some sort of unique upper motor neuron lesion could result in hypotonia.