she's talking about or talked about

I  walked to the nurse desk and asked for a bottled water and one of the nurses told me that I couldn't take any liquid before surgery. Instead, she told me to get some rest. I walked back to my room and told my mom. Mom said, "Sounds like she knows what she's talking about."

You're using direct speech (the sentence in bold).  This means that what you are talking about happened not long ago and therefore the present continuous tense is fine. When someone asks you to have some food and you have eaten earlier, you will turn down the offer, saying, "No thanks. I have eaten."  You've to use the present perfedt tense, not the past perfect tense.

I hope this gives you a clear picture why the present tense is used for something which has taken place recently.

N2G,

The expression "X know(s) what (I/he/she/they) (am/is/are) talking about" is idiomatic. You don't have to be actually speaking at that very minute. It just means that the person is knowledgable.

You should go ask Dad. He knows what he's talking about when it comes to cars. -- It doesn't mean he's talking now, or even that he's spoken about cars any time in the last week, but when he does talk about them, he's right.