It didn't say "apprentice" though, just protégé. Which might very well mean that he used to be his apprentice. Cristos can't be that young any more or his appointment to the SC would be an absolute joke. So maybe it wasn't polite to call him someone's apprentice.
Agreed, though given the Centuries long timeline we're talking he could have become his protoge /after/ he gained full Council Status. Apprenticeship seems to last a decade or so at most, during a wizards first 2-3 of them. Meanwhile they arent given full respect until they've reached a century of membership (as denoted by the stole colors), so that's 70+ years that it would make sense for older Wizards to take the younger (but still full members under their proverbial wings).
Similarly, in peacetime at least you'd need to be a full member before joining the wardens, so a protoge could accurately describe a quasi-Master/apprentice relationship during that phase of training.
Hell, in SK or TC they were arguing that McCoy had not been with Harry long enough to call him his Apprentice, so protoge would be the next logical term
All that to say that Apprenticeship is a far more specific and formalized relationship to the WC, whereas protoge is a synonym being used in a more generic way.