```python
def drop_small(counts: dict[str, int], threshold: int) -> dict[str, int]:
    """
    Drop keys from the dictionary where the value is less than the given threshold.
    The function modifies the input dictionary in place and returns it.
    
    :param counts: Dictionary with string keys and integer values.
    :param threshold: Integer threshold for dropping keys.
    :return: The modified dictionary with keys dropped.
    """
    if not isinstance(counts, dict) or not all(isinstance(k, str) and isinstance(v, int) for k, v in counts.items()):
        raise ValueError("counts must be a dictionary of strings to integers.")
    
    # Use a set to track keys to delete
    keys_to_delete = {k for k, v in counts.items() if v < threshold}
    
    # Delete keys from the dictionary in place
    for key in keys_to_delete:
        del counts[key]
    
    return counts
```