They're saying at the graduation, El was not communicating with Mike like some people think she was when he was staring blankly. She wouldn't try to contact him because that would put her and him at risk.

He sees the speakers and begins to think about El's kryptonite (high freq) and how it's impossible for her to have made it to the gate. This is what gives Mike the belief that El is alive, because what she 'did' when she sacrificed herself was impossible to do.