The two sentences are often used interchangeably and you should never assume a literal distinction.
However, in the literal sense, "I am not talking science here" would mean, "I am not using scientific jargon that you would not be able to understand," while "I am not talking about science here" would mean, "I am not talking about the scientific subject, I am talking about common knowledge and/or common sense."
One SHOULD be used in situations where you want to explain that you're using simple terms so it should be easy to understand; while the other simply informs that you're talking about a simpler subject.
Both meanings are close enough that they are ambiguous even when used in their specific ways, so you should not ever assume that someone is using the correct and very specific meaning of either sentence.