Being able to find such sentences from the internet which deviate from the norm does not mean you are right.
In schools and exams, there could be two or more answers to the same question but only the best answer is marked correct.
It is arguable whether the alternative answer is correct or grammatical. Correctness has a lot to do with following current norms.
In this case, the most widely accepted preposition after thank is 'for' and not 'that'.
Do a search again for 'thank somebody for' and compare the results.
P.S. It does sound more logical that 'you thank somebody for something' rather than 'you thank somebody that something happened'.
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Student or Learner