They are back to back sentences. Pretty bad attempt at obfuscating. The first sentence claims data “across” all channels, and the following sentence breaks it down.
“Across the board” means something closer “true in every case”. Though as I type this, I realize some people my think of it as “in total.”
“across the three major cable news networks”
Which I paraphrased as
“Across all the channels”
Which the person replying to me changed to
“Across the board”.
It's specifically not "across the board."