They just change the source of the ATT information from IRS 1 or 2 to 3. With this procedure they can compare the 3 IRS - and found out that they all show the same thing, so it might have reassured them, at least it didn't resolve their confusion.
It's not a "one way button" where you go to attitude only information, or have to calculate variation/drift by yourself. This button also exists, but is placed on the overhead panel, close to the ADIRS. It wouldn't have helped in this scenario.