Schulz was indeed sincerely praising Qiao Yu.
In fact, whether this sentence is a compliment or a regret, it depends on the age group.
If Qiao Yu were already a young man in his twenties, then Schulz might have regretted that Qiao Yu wasted too much time, resulting in an unstable foundation. This would make him struggle greatly in independently researching some fundamental problems or developing a new mathematical framework, possibly leading to efforts ending in failure.
But Qiao Yu is only sixteen years old.
This means that Qiao Yu still has plenty of time to learn and make up for his weak foundation, allowing him in the future to tackle world-class problems, or even create his own mathematics. Just like what he is currently doing.
The terror of age advantage lies here; Schulz even feels that it doesn't matter if Qiao Yu wastes another two years, since he's still so young.
Qiao Yu just feels very ashamed.
