If I'm reading this correctly the investigative reporters can't be sued because they did not actually splice into any wires.
What he is saying is that they can't be sued (or prosecuted) because the ACORN employees were taking notes and writing stuff down during the conversation and by taking notes they were themselves "recording" the conversation. Handwritten notes would be considered an interception of a conversation in accordance with the definitions in the MD Code. A pencil or a pen would be considered "mechanical, or other device" under the statute.
On top of that the conversation was not "private" because the door was open and other people were within earshot (such as the children making noise in the background).