I have a theory about why. Notice there are no diodes used for any other purpose in that circuit? Using transistors for diodes means one less part in inventory, and that's worth money in a production line factory.Q5 and Q6 are acting as clipping diodes - which is unusual,
An extreme case is represented by a place I worked which used 1% resistors in several circuits, but the factory did not stock all 96 values per decade. It would require 576 bins to hold all the values from 1 ohm to 976K. Lots of money saved by only stocking the 2 or 3 dozen values actually used in their circuits.