It's very possible that Supermassive owns the source code to the original game and wouldn't make that available, but I think you're missing the larger point here. Whether it was a remake or a remaster doesn't necessarily mean that it's going to be in 60 fps. The original game runs at 30 fps and there was never a 60 fps patch.
Oh they could have used decima? I'm sure that thought never dawned on them (no pun intended).
Just because the previous game used decima doesn't mean you can easily compile the old game against the latest version of decima.
The reality is this is a start up and very few people who work there have probably ever used decima, let alone the latest version of decima. I love how people act like decima is this well used engine across the industry or that Guerrilla Games has the same support level for it that Epic Games has for Unreal Engine.
In its entire history, 4 teams have used Decima. Guerrilla being the primary, Guerrilla Cambridge (1 time) that shut down, Supermassive (1.5 times), and Kojima Productions (2 times).
Kojima likely used it because it would be vastly cheaper than using Unreal Engine and he has a close relationship with Herman Hulst. Kojima Productions was filled with veteran devs used to using engines that were not broad based engines. Konami built custom engines for all their games. It wasn't until the Fox Engine did they really push to develop a consistent engine. I'm sure Kojima has also helped to develop Decima.
Decima is not a broad use engine meant to compete with Unreal Engine.