I didn’t watch but the fact of the matter is the game has progressed frighteningly quickly over the past 10/15 years, and that counts as double for mid tier European teams like Leipzig, Dortmund, Spurs etc. The amount of money and sport science available to these teams is insane.
Doesn’t matter how good Celtic get or how much fitter/faster they are from the John Hartson days, they’re miles behind these teams (never mind your Madrid’s or Man City’s) and won’t ever make up that ground.