That depends on how the seeds are generated. But I am sure it would be random and not selected from a list. And the probability of having the same seeds randomly is almost 0.
Almost 0 doesnt mean that it is imposible because there is still a posibillity for that. He might have been lucky enough to notice it and exploit it further. Although this all is a pure speculations and theories on how he did it, it hasnt been proven yet that two account rolling with the same seed could generate the same server seed.
I can 100% guarantee that is not possible. When seed is generated the same seed can not be generated on another account, its impossible. My best guess is he duplicated 2 same accounts and thats how he got a hold of same seed.
But stunna already said that his accounts have the same seed that is how he cheated pd