QModbusTcpClient::connectDevice() always returns true

#include <QApplication>
#include <QDebug>
#include "modbusclient.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    ModbusClient client;
    qDebug() << client.open("192.168.1.157", 502); // address of a non-existent device

    return a.exec();
}

#ifndef MODBUSCLIENT_H
#define MODBUSCLIENT_H

#include <QObject>
#include <QModbusClient>

class ModbusClient : public QObject
{
    Q_OBJECT

public:
    explicit ModbusClient(QObject *parent = nullptr);
    bool open(QString host, int port);

private:
    QModbusClient *_modbus;

};

#endif // MODBUSCLIENT_H

#include "modbusclient.h"
#include <QModbusTcpClient>
#include <QVariant>

ModbusClient::ModbusClient(QObject *parent) : QObject(parent)
{
    _modbus = nullptr;
}

bool ModbusClient::open(QString host, int port)
{
    if (_modbus)
    {
        _modbus->disconnectDevice();
        delete _modbus;
    }

    _modbus = new QModbusTcpClient(this);
    _modbus->setConnectionParameter(QModbusDevice::NetworkPortParameter, port);
    _modbus->setConnectionParameter(QModbusDevice::NetworkAddressParameter, host);

    _modbus->setTimeout(250);
    _modbus->setNumberOfRetries(1);

    return _modbus->connectDevice();
}