The code looks rather complicated. Instead of trying to understand it, I'd just like to point to NickC's answer to the linked SO question.

If I add **kwargs to his optional_arg_decorator like this...

def optional_arg_decorator(fn):
    def wrapped_decorator(*args, **kwargs):
        if len(args) == 1 and len(kwargs) == 0 and callable(args[0]):
            return fn(args[0])

        else:
            def real_decorator(decoratee):
                return fn(decoratee, *args, **kwargs)

            return real_decorator

    return wrapped_decorator    
    

...and adapt your decorators like this, I'm getting the same output from the test cases.

from functools import wraps

@optional_arg_decorator
def my_text_decorator(f, start_val="-=[", end_val="]=-"):
    @wraps(f)
    def wrapper(*args, **kwargs):
        return "".join([f.__name__, ' ', start_val,
                        f(*args, **kwargs), end_val])
    return wrapper

@optional_arg_decorator
def my_text_decorator2(f, start_val, end_val="]=-"):
    @wraps(f)
    def wrapper(*args, **kwargs):
        return "".join([f.__name__, ' ', start_val,
                        f(*args, **kwargs), end_val])
    return wrapper
    
    

The code looks rather complicated. Instead of trying to understand it, I'd just like to point to NickC's answer to the linked SO question.

If I add **kwargs to his optional_arg_decorator like this...

def optional_arg_decorator(fn):
    def wrapped_decorator(*args, **kwargs):
        if len(args) == 1 and len(kwargs) == 0 and callable(args[0]):
            return fn(args[0])

        else:
            def real_decorator(decoratee):
                return fn(decoratee, *args, **kwargs)

            return real_decorator

    return wrapped_decorator    
    

...and adapt your decorators like this, I'm getting the same output from the test cases.

from functools import wraps

@optional_arg_decorator
def my_text_decorator(f, start_val="-=[", end_val="]=-"):
    @wraps(f)
    def wrapper(*args, **kwargs):
        return "".join([f.__name__, ' ', start_val,
                        f(*args, **kwargs), end_val])
    return wrapper

@optional_arg_decorator
def my_text_decorator2(f, start_val, end_val="]=-"):
    @wraps(f)
    def wrapper(*args, **kwargs):
        return "".join([f.__name__, ' ', start_val,
                        f(*args, **kwargs), end_val])
    return wrapper