Normally if you have an end matching a function statement then the workspace is static and nothing can be added to it dynamically by assignin('caller') such as is used by syms. syms is not a keyword: it is an ordinary function that uses assignin to create the variables.
It appears that the case of a function inside a script might be an exception that is not considered static, unless the function has a nested function.
The assignment to the variable provides enough transparency that the static workspace mechanism is able to decide that the variable should exist.
However you should not count on this continuing to work exactly the same way: it is likely that at some point MATLAB will be changed so that only variables plainly assigned before the function call are considered to be known, or perhaps only variables plainly assigned to before the sub-function definition. Mathworks has been actively making the rules stricter recently.
