Function "overloading" with only one function implementation in Python

In this Python code (the algorithm itself isn't relevant to my question):

def test_balanced(s):
    # Check the parentheses in the given string are balanced
    count = 0
    for c in s:
        if c == '(':
            count += 1
        elif c == ')':
            if count == 0:
                # Close before open
                return False
            count -= 1
    # Check all open have been closed
    return count == 0

def remove_odd_brackets(s):
    if test_balanced(s):
        return {s}
    
    # Find positions of opening and closing parentheses
    os = [i for i, c in enumerate(s) if c == '(']
    cs = [i for i, c in enumerate(s) if c == ')']
    
    # Progressively remove more parentheses until we find valid solution(s)
    for n in range(1, len(os + cs)):
        result = []
        for c in combinations(sorted(os + cs), n):
            test = [ch for i, ch in enumerate(s) if i not in c]
            if test_balanced(test):
                result.append(''.join(test))
        if result:
            return set(result)
    return {''.join(c for c in s if c not in '()')} 

I don't understand how is it possible to call the function test_balanced(s), first with s as a string, and then as a list of strings of length 1 each.

Is it a strange example of function overloading? (strange for me because there is only one implementation of the function...)

I do not understand what the interpreter is doing in this case "behind the scenes" (briefly).

Thanks in advance!