TM yes I was talking about the corrosion on the inside surface of the shaft, not corrosion in the drill hole which as you say is obscured by the plug and probably difficult to see even if the plug is removed. However the photos that were published showing the corroded shaft internals against a shaft with internals in good condition (mirror machined finish) surely indicates that corrosion to that extent was easily visible?
I just checked the photos again (SIN 2600-S-00 on TIPI) and it doesn't make it clear what level of magnification is being used, however I wouldn't have thought it too difficult to visually check for corrosion with the appropriate degree of magnification.