I'm guessing this is safer than attacking other countries
If this is true and considering the source it's a big if then it has to be safer than doing the same to someone else say China.
The Chinese have an army of cyber warfare agents ready to respond in kind. The North Korean's due to the local limits are unlikely to have a similar set up. I'm sure they have some form of cyber warfare specialists but the numbers must be on the low side compared to just about any other nation on earth.
Considering most of the country doesn't even have access to electric lights I'm guessing no one noticed anyway.