Chalk said that he is a writer.

Is gramatically right? as given in the study material for conversion of tenses.

The sentence given is correct. There is a rule in the english grammar which mentions that when we convert a sentence from direct speech into indirect speech, a simple present tense becomes a simple past tense.

However this sentence is an exception since it conveys a fact that the chalk is a writer. When a fact is conveyed in direct speech then it remains unchanged within indirect speech. Therefore the tense has not changed in this sentence and the sentence is appropriate.