DMCA takedowns can be abused, this is not what this is.
DMCA is open to abuse in that it can be used to take down content which never infringed any copyright, with no repercussions to the entities filing takedowns illegitimately.
Santo Campo own the copyright to their own work, and Pewdiepie's usage of their copyrighted content isn't fair use. When you own the copyright to something you do have the power to arbitrarily control what is done with it for any reason. That isn't abuse, that is the law functioning exactly as intended.
Whether the law should be like that is another debate, but this is the current situation. (In my opinion, the DMCA has a lot of problems and should be abolished for reasons beyond this thread.)
We already know you don't need a reason to DMCA someone, that's why Digital Homicide DMCA'ing Jim Sterling for saying thing they didn't like, it's why Alex Maur DMCA'ing over 100 Youtubers for playing or critiquing a game she didn't own the music to, it's why Campo Santo is DMCA'ing PDP for saying thing he didn't like with the lovely added bonus of it being over a game they're not the developers of.
Firewatch has every right to dissociate themselves from PewdiePie because of this, that doesn't mean this isn't an abuse of the DMCA.
Digital Homicide were abusing DMCA as Jim's usage of their content for short impression videos with critique was fair use.
Pewdiepie's usage of Campo Santo's game in a let's play is not fair use.
Copyright says nothing about what reasons you are allowed to have for doing certain things with you copyrighted work. It simply dictates under what circumstances another piece of work is considered derivative and not fair use.
Copyright holders can arbitrarily dictate what happens to derivative works, and do not need to provide reasons for doing so. This is how the law is designed to work (and how it should work IMO.)