The assumption is only valid locally.

Author: benjie

spec/Section 5 -- Validation.md

@@ -1319,8 +1319,9 @@ which may be a variable usage. The
 [All Variable Usages Are Allowed](#sec-All-Variable-Usages-Are-Allowed)
 validation rule ensures that each {variableUsage} is of a type allowed in its
 position. The [Coercing Variable Values](#sec-Coercing-Variable-Values)
-algorithm ensures runtime values for variables coerce correctly. Therefore we
-can assume the runtime value of each {variableUsage} is valid for usage in its
+algorithm ensures runtime values for variables coerce correctly. Therefore, for
+the purposes of the coercibility assertion in this validation rule, we can
+assume the runtime value of each {variableUsage} is valid for usage in its
 position.
 
 The type expected in a position includes the type defined by the argument a