Think of it as this. You watch athletics for years. Suddenly some guy runs the 100meters under 8.5 seconds. A regular shaped sprinter, not like phelps in terms of swimming.
Don't think this is the case for modern football.
Training methods during the week can be modified, the only must is 2-3 matches of 90 minues per week.
No way. Take out Courtois, and it's a different story. Remove their style of play, and you'll see it's less about defensive marking and more about a collective aggression.