I have no idea what game you were watching because you obviously didn't see the mismatches created due to the height differential.
In their first game, Horford & Morris combined for 35 points & 10 rebounds. Last night, both Celtic centers were out, meaning they had to play a small lineup the entire game. Then there's the fact that Hayward is just coming back from an injury. In the last game, against the Bucks, he put up 18. Last night, 11 points, 3-11 from the field.
Their defense was really hamstrung having absolutely nobody in the middle who could even try to change any Giannis shots.
But, you're entitled to your opinion.