It's coming with a story DLC. And, it probably accounts for the typical $10 price increase between a brand new PS4 game and a brand new PS5 game. This is more than just patching it (they already unlocked it to 60fps/4k with a free patch, just like Days Gone, Last of Us Part 2, and God of War). It's likely built-in, due to the fact, they don't want people with the PS5 just buying the PS4 directors cut at $79.99 CAD and then getting the free upgrade. It sucks, but Sony would either need to back down from their pricing for PS5 games or raise the PS4 one (which will lead to outrage on that side).
Activision did it on Tony Hawk, at least for PS5.
Sony and Microsoft have dramatically different game plans on how they plan to maximize revenue this generation. Sony still wants you to buy the games, Microsoft wants you on Gamepass, and hope you like the in-house studio games enough to buy DLCs. I will give credit to Microsoft though, the recent Doom Eternal upgrade was free for PS5. But, if you want one of the two Ancient Gods DLC's even with Game Pass its 23 bucks (CAD) per DLC ($25 without on Microsoft platforms and $27 on Playstation), which is about in-line with Ghost of Tsushima's price upgrade, unless the DLC is ridiculously short (but most of the recent Sony in house DLC's have been quite sizable such as Horizon:Forbidden West and Spider-Man). Historically, American companies tend to be more consumer-friendly than Japanese ones, partially due to cultural differences (which is reflected in Sony and Nintendo's refund policies, and led to the nightmare Cyberpunk refund situation with Sony).