Case sensitive?!?!?!? What is this, 1970 Unix?
I don't know for certain, but you might actually not be too far from the truth. I am a Windows user and have only briefly dabbled in Linux, mostly Ubuntu (the user-friendliest form of Linux) and that operating system is case sensitive for EVERYTHING, with no exceptions of which I was ever made aware.
So the fact that whatever Factorio uses as its parser environment is case sensitive isn't entirely surprising to me (but I will say in your defense that if I were trying to kitbash and hack my mods into being able to use them in v0.18, I would have been just as ignorant as you that this example is also case sensitive.)