Interestingly botnets are probably a month away from being unprofitable for mining Bitcoins, in fact GPUs will be unprofitable for mining Bitcoins all-together. ASIC devices are being shipped over the next six months which will increase the mining difficulty substantially.
I think the comment is referring to opportunity cost, since botnet's can also be used for other profitable activities, which may bring in more money than bitcoins (such as renting them to interested parties).
