"He said that when it comes to movies, there are so many genres."
"Speak" goes into the past tense. But the "rule" (to put present simple into past simple) doesn't always apply. Where the general truth of a statement is still generally true, it should stay as present simple. Especially if the original statement was not about a specific person or unique situation (but even then, it is still sometimes best to keep it as present simple when reported).
Einstein: "Time and space are relative." --> "Einstein said that time and space are relative." It would be a bit strange (but not technically wrong) to say: "Einstein said that time and space were relative."
You are right to wonder about this. Remember, there are exceptions to almost every rule.