[Python] Why is it that I "cannot use assignment expressions with comparison" in Python?

In the examples below, x is assigned using the walrus operator and is then printed.

mystring = "hello, world"


#if 1
if x := mystring == "hello, world":
print(x)

#if 2
if x := (mystring == "hello, world"):
print(x)

#if 3
if (x := mystring) == "hello, world":
print(x)

#if 4
if "hello, world" in (x := mystring):
print(x)


The output:

True
True
hello, world
hello, world


I believe that I understand how each of these if statements works:

1. 
   #if 1 and #if 2 are the same thing where the string comparison == returns True, and that value is reassigned to x.
   
   
2. 
   #if 3 assigns mystring as x before checking the truth of this equal to "hello, world".
   
   
3. 
   #if 4assigns mystring as x then checks if "hello, world" is in this string.
   

So what about this final #if 5 is ambiguous to Python such that it gives a Syntax Error?

#if 5
if "hello, world" in x := mystring:
print(x)


SyntaxError: cannot use assignment expressions with comparison


Is this related to PEP 572 Exceptional Cases:

Unparenthesized assignment expressions are prohibited at the top level of an expression statement.


there is no syntactic position where both are valid.
​

In reality I am looping through the pathlib Path.iterdir() generator object that yields strings with .stem. This does not change the result:

for file in folder.iterdir():
x := file.stem == "mystring":
print(x)

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