No that is not how it works. A slave can send data but it can not initiate a data transfer. So a slave does not turn into a master to respond to a request for data.
A Slave like UNO-2 of Post#8 can initiate data transfer by turning itself into a Master. In the example of Post#8, the Slave (UNO-2) sends the ACK (0x06) in respone to Motor-1 command by executing the following instructions in the Wire.onReceive(receiveEvent) subroutine.
void receiveEvent(int howmany)
{
sei();
if(Wire.read() == 0x01) //Command to Move motor-1
{
Serial.println("Motor-1 Move Command Received");
//--SEND ack (0X06)--
Wire.beginTransmission(0x50);
Wire.write(0x06); //ACK Send
// Wire.endTransmission();
//---Motor-1 move command
digitalWrite(13, HIGH);
delay(5000); //5-sec delay to move slider
digitalWrite(13, LOW);
Serial.println("Slider Moved!");
Wire.endTransmission(); //UNO-2 is generating the SCL pulses
}
}