Really depends on what he told them. If he gave them the Packer calls, then they are cheating. If he told them how to read a an offense pre snap, then it might be legit. The article doesn't say if he gave them the checks, or how to know dummy calls. I kinda doubt he did that. Would be useless during the season.
I love that he is doing this, but I hope is systemic and an interactive approach. If its a one off, noticed this on film, here is a hint, then it might be of limited use during the season. Might even get them caught looking for a certain play or player. If he is teaching the defense how to read the offense like the O normally does in reverse, it might lead to better audibles if the Packer D alignment calls can be used quickly.
Could also help the D get into the most favorable alignment without making a call. If you don't want the Seahawks to pull in a TE to block, you might refrain from making an alignment adjustment because your film study plus knowledge of the QB tells you that will happen if you drop a safety to a Robber spot versus 1 RB, 2TE, 1 WR