what is this IC and pin on the HC-SR04 which is being used for "Raw Echo" data?
The Raw Echo lead looks to be attached to pin 10 of the EM78P153S microcontroller which is used to process the signals from the ultrasound receiver circuit of the HC-SR04. A second article with a similar modification explicitly mentions pin 10 however it identifies the actual IC component as U1. The mbed schematic identifies the EM78P153S as U1.
The purpose of this modification is to bypass the signal processing of the EM78P153S and pull the raw signal generated by the ultrasound receiver circuit into another microcontroller, an Arduino in this case, to process the raw signal.
The HC-SR04 schematic https://os.mbed.com/media/uploads/4180_1/hcsr04_schematic.svg identifies U1 as an EM78P153S microcontroller.

This article, https://www.instructables.com/Enhanced-Ultrasonic-Range-Finder/  showing a similar modification mentions pin 10 of U1 as the site for the modification and contains additional information from testing various ranges and with two objects. An oscilloscope was used to trace the raw signal showing the following waveforms. CH1 is from the Echo lead of the HC-SR04 and CH2 is from the Raw Echo lead attached to the pin of the IC.
The initial signal detected on CH2 appears to be the Receiver picking up the ultrasonic burst from the Emitter. The returning echoes seen as as multiple, closely spaced signals on CH2 indicates the echoes are composed of multiple sound waves detected by the Receiver. CH1 show that after a delay so as to eliminate the Emitter's output, the Echo lead is raised to HIGH and held HIGH until a rising edge from the Receiver circuit is seen at which point the Echo lead is then dropped to LOW.

The Raw Echo signal shows a rising edge when the echo is received with a Vmax of 4.079v which is sufficient to indicate a HIGH on a 5v GPIO pin.
The Arduino source code used for measuring two echoes is as follows. The first object is detected using the Echo lead of the HC-SR04 per standard practice. Once an echo is detected, the code then looks for a second echo by waiting on a HIGH from the Raw Echo lead. Notice the wait between when the first echo is detected on the Echo lead and then checking on a HIGH from the Raw Echo lead. This wait is used to eliminate any of the multiple ripples from the first echo detected as an echo is not a single sound wave but rather multiple, closely space sound waves which causes the Receiver circuit to generate multiple, closely spaced signals.
  // ----- generate 10uS start pulse
  digitalWrite(Trig, HIGH);
  delayMicroseconds(10);
  digitalWrite(Trig, LOW);

  // ----- wait for pulse(s) to be sent
  while (!digitalRead(Echo));                 //wait for high
  start_time = micros();

  // ----- set timeout radius
  timeout = start_time + 12000;               //set timeout radius to 2 meters

  // ----- measure first object distance    
  flag = false;
  while (!flag)
  {
    if (!digitalRead(Echo)) flag = true;      //exit loop if object detected
    if (timeout &lt; micros()) flag = true;      //exit loop if timeout exceeded
  }
  finish_time = micros();

  // ----- calculate first object distance
  time_taken = finish_time - start_time;
  Distance1 = ((float)time_taken) / 59;

  // ----- wait for first object echo to finish
  pause = finish_time + 1000;                 //1000uS means 17cm closest object spacing
  while (pause &gt; micros());                   //wait 1000uS (delayMicroseconds(1000) erratic)

  // ----- measure second object distance
  flag = false;
  while (!flag)                               //wait for high
  {
    if (digitalRead(RawEcho)) flag = true;    //exit loop if object dectected
    if (timeout &lt; micros()) flag = true;      //exit loop if timeout exceeded
  }
  finish_time = micros();

  // ----- calculate second object distance
  time_taken = finish_time - start_time;
  Distance2 = ((float)time_taken) / 59;

If this source code were modified to allow for more than two objects being detected, the source code would need to incorporate logic similar to that used for debouncing a simple pushbutton switch.  The rising edge would be detected and then any other changes seen on the pin would be ignored for some time period. From the oscilloscope trace, it looks like 1 millisecond should be more than sufficient.