Perhaps the author likes to start from 1.8V than 1.2V.
One may think the diode is added as a protection from a possible reverse polarity since the Veb (from E to B) breakdown is usually low. In this case it is better to use a diode with a higher breakdown voltage than 100V.
On the other hand, it is better to add a resistor at the entry base as 1K or 2K2 for example.
Kerim
Edited:
Since the 4 limiting current resistors have the same value, one may notice a slight brightness differences. LED_1 has the highest brigthness, LED_4 the lowest due to the higher voltage at the collector of its transistor.