I think what they meant was that with most newer cars (like 2000 and newer) you can't reset the ecu by just disconnecting the battery but have to hook up a reader or take it to the dealer to have them reset it. I don't know if they're talking about error codes and turning off the check engine light, etc. or just resetting it enough to allow it to "re-learn".
I kind of agree because of past experience. I had a 1998 Dodge Neon and had something happen where the check engine light came on. To reset it, all you had to do was disconnect the battery for a little bit. My dad went and bought a 200 Neon and had the check engine light come on because he didn't tighten the gas cap good enough and tried to reset it by just disconnecting the battery and it didn't work. He had to take it to the dealer and pay to have it reset. The dealer said that with the newer cars, you have to take it to the dealer to get the ecu reset and can no longer do it by just disconnecting the battery.
Is this the same situation with the WRX's (I have a 2004)? Never had the check engine light come on so I don't know. I have had a few times where I had the battery disconnected for a time (recently while changing the spark plugs) and noticed that it drove a lot better afterwords, but thought that was just because of the new plugs.