This is.... actually kind of a smart chess play Rutherford is doing here. Quality there is not 1 for 1, maybe the full deal isn't released. but if it is, here are a few of my thoughts.
-Trying to get younger: Matheson will be younger at the end of his deal than Hornqvist is currently now. Hornqvist age has been really showing the past few seasons with extended injuries and will be 36 in the final year of his contract. Not sure having a 36 year old who's body is completely destroyed making 5.3million. I love Horny to death and thankful for everything he has done for this team, but he's at the point where I'd rather play against him 3 times a year vs. watching him play 50-60 games a year.
-"Saving" cap space: 425k in cap space is being saved in this deal for the next 3 years. Granted, not a world of difference, the Pens are rumored to not be a cap team next year and I'm guessing, especially with giving Bjustad away for nothing other than just saving 2 million, this is the main aim for the deal.
-Free Agency players: On contradiction to my last point, The Pens do want to clear up as much cap space as possible to be players in Free Agency as possible. With a stagnant cap, chances are for the middle tier free agents, price will stay the same. Any $1 helps.
-Changing it up: Listen, the definition of "Crazy" is doing the same thing over and over again and expecting different results. Something like this was bound to happen while the Pens are trying to blow up the team. I'm sure GMJR is just shopping Horny around and found someone who would take him. If Matheson is the best we could do, so be it. It helps in the Previous three areas I mentioned.
Lastly, and a point most people are really looking over,
-Expansion Draft: Hornqvist carries a NTC, which means he would have to be protected in the upcoming expansion draft. By moving him now, we will have the ability to protect someone else without being forced to protect a 34/35 year old, or paying Seattle to take someone else.