Horvat deal was Beauvillier (traded to Chicago for a 5th), Raty (2021 2nd round forward), and a 1st. But it was also a month earlier in the season and with 25% retention.
Given Carolina has most of the cap space to make this move ($5.3M as of today of Guentzel's $6M cap hit), there isn't a need to both send an expensive roster player back and have 25% retention.
Would Carolina do a 1st and a prospect for Guentzel? I don't think they would for a rental, but he would fit well. Would Pittsburgh prefer a young roster player (Drury) and a 1st given they are still trying to retool around Crosby, Malkin, Letang, and Karlsson?