The temperature effect is small because all diodes Q1 = 8 in parallel and Q1 have exactly the same temperature coefficients (say 2 mV/deg).
The current through diodes is small when it flows through R4 only and higher when the current is supplied also through MP1 , R5 .
Let us assume that R1=R2 ( I do not know if this assumption is right but there is no value on the drawing)
For low current Vx is < Vy (the current through single diode of Q2 is 8 times less than through Q1 so voltage must be lower) an additional voltage on R3 (I assume that its value is very low) is also not significant.
When MP1 change its state to a conductive one, the currents will increase, the voltage DIFFERENCE between Q1 and Q2 remains the same but the voltage drop on R3 will became significant hence Vx > Vy and this will change the comparator output.