问题是您需要从查询中获得一些结果。阅读有关生成查询结果的信息。
修改方法如下

public function verify_password($username, $password)
{
    //use method chaining, it's more efficient and less typing
    $query = $this->db
        ->select('password')
        ->from('user_account')
        ->where('username', $username)
        ->get();

    //were any matching rows found?
    if($query->num_rows() > 0)
    {
        // get first row of data and check 'password' value against passed value
        // return is a boolean
        return $query->row()->password === $password;
    }

    //There are no rows that match so clearly not logged in
    return false;
}

正如问题中的注解所指出的，您不应该存储纯文本密码。