I always find this one funny. Before Ange came in Celtic had won, what, 9 out of the previous 10 titles? Celtic have the biggest budget by a wide margin over everyone including Rangers. They are utterly dominant other than a single blip of a season. Shambles is an overstatement.
Sure, it's a pressurised job but winning trophies with Celtic is never an indicator of an elite manager.
There is context. The season before he arrived, Celtic finished second, twenty five points behind Rangers. Celtic finishing second is finishing last at the best of times, but by 25 points, it is a catastrophe. He then won the league in his first season by 4 points. That's a 29 point turnaround in a season with a team that he was still building.
As has been said, the manager needs to make the team better than the sum of its parts. He did that- and then some- at Celtic.
The man knows to how to manage, whether that will result in trophies at Tottenham remains to be season.