Difference between $true and $false under the hood
What's actually happening here is that the parser is seeing -eq $true and -eq $false as arguments being passed to the functions.
To see that this is the case, add [CmdletBinding()]Param() to the functions like this:
function returns_true(){[CmdletBinding()]Param() return $true}
Better explanation as to what's happening:https://stackoverflow.com/a/49115149/9446818
Parentheses help. What you really want is:
if ((returns_true) -eq $true){and
if ((returns_false) -eq $false){