Arduino will receive and interprete correctly 3.3V signal.
I mean the 3,3V TX from the RF module won't get shifted to 5V by the 74HC4050N, right? So what's the point of this connection?
Wire.begin();digitalWrite (A4, LOW); // turn off internal pullupdigitalWrite (A5, LOW); // turn off internal pullup
You are quite right it is a totally useless circuit designed by an idiot.It will not protect the 3v3 system.
... which enables the device to be used in HIGH-to-LOW level shifting applications.