Value isn't determined in a vacuum, as you are trying to do. Edmonton was extremely deep at forward with young talent: McDavid, Eberle, Puljujarvi, RNH, Draisaitl, Lucic, etc.. and very weak on defense. It's been their Achilles heel so they they traded from a position of strength (forward) for a position of weakness (defense).
Carolina has the opposite problem. They are very strong on Defense (Hanifin, Faulk, Hainsey, Pesce, Slavin, Murphy, Fleury, McKeown, Bean, etc...) and very weak on offense. So it would be silly to trade from a position of weakness FOR a position of strength.
Teams make trades not to get the most value, they make trades to improve their team. Edmonton is a better team today by losing Hall and gaining Larsson (who I think is much more than a "mediocre" defenseman). If the Canes traded Skinner for a 3,4 Dman, they would be a much worse team. Therefore, as of right now, yes, Skinner's value is MORE than a 3/4 defenseman, simply because the Canes would never accept that in a trade. That may change next year, or the next, or the next, but right now, it's really not more complicated than that.