If pt1 and pt2 both point to the same instance you can test them for equality. You're misreading what that exceprt says.
When they said "comparison" they were referring to something like if(pt1 < pt2) which is actually useful in an array, but not really anywhere else. "Comparison" is not the same as "equality or inequality".
Here's something from a little newer reference.
From ISO C99:
6.5.9 Equality operators
Syntax
1 equality-expression:
relational-expression
equality-expression == relational-expression
equality-expression != relational-expression
Constraints
2 One of the following shall hold:
— both operands have arithmetic type;
— both operands are pointers to qualified or unqualified versions of compatible types;
— one operand is a pointer to an object or incomplete type and the other is a pointer to a
qualified or unqualified version of void; or
— one operand is a pointer and the other is a null pointer constant.
