Regardless of what the Trump administration will tell you, that's not it's name. The executive branch is not empowered to unilaterally change the name of a department.
Even by ignoring the name change, that is its function. Even if it was called department of defense, it's actually department of war.
Regardless of what the name legally is, they are in fact initiating war against other nations and Palantir is one of the main players in those wars.
If it's what they call themselves and what they're currently doing, how much does it matter what the official name is?
It’s not empowered to unilaterally declare war without approval from congress, either. But here we are.