Using the definition of "hack" from a dictionary? You are going to come up with a whole bunch of different meanings, and even different interpretations of the meaning. In your own words, please "use some intelligence", and read the context that IS was using it in instead of nitpicking over the meaning of the word.
IS is saying that there was, according to their system, no unauthorized access. The "hacker" obtained the username and password without compromising the core IS system (no brute force attack etc.).
This probably means the admin account itself was compromised, not the system, probably due to some successful phishing. That or the admin had a keylogger/trojan on a machine he used to log in. Of course, it is also possible, if quite unlikely, that the guy bypassed every single security measure IS had in place without leaving a trace.
So no, strictly speaking, if what IS is saying is correct, the hacker did not use his 1337 n1nj4 5k1llz (read: programming skills) to hack into their system. Wouldn't want to be the admin in question....
Regardless, the method itself does not really matter. Whether it was a hack or not, the end result is that the system was compromised, and whilst the damage to the system itself was minimal, the leaked information is going to cost them. So IS had to respond to try and put the minds of their existing customers at ease, even if it is playing on the meaning of the word themselves.