It’s a reference to an old reddit post. In the post, the OP explained they had a knife at their toilet for poop that got stuck, hence the poopknife. It was only later in life when they asked a friend for their “poop knife”, when they discovered that nobody else has a knife like that and how weird it is.
Are you sure about Hell Let Loose not working on Linux on your system? The ProtonDB page has many reports if HLL working fine since about a month.