Reason of high packing density in integrated injection logic family
The promise of Integrated Injection Logic (I2L) was a bipolar circuit technology that could achieve LSI density: the speed of bipolar with the density and low power consumption of CMOS. It didn't manage to overtake CMOS, but it's still an interesting technology historically.

The density problem with TTL logic is it uses lots of resistors. On an IC die, a resistor is bulky, consisting of a long, narrow diffusion region snaking around. I2L instead uses a lateral PNP transistor as a current source to inject the base current, avoiding the use of resistors. In addition, the PNP transistor and the NPN transistor for the gate can be combined into a cell without additional wiring, providing a compact layout. (Thus, it has an "integrated injector".)

A detailed explanation of I2L with diagrams is in the paper you mention: "Integrated injection logic: a bipolar LSI technique". You can compare with TTL layout discussed in my article on the TTL 74181 ALU. 

One confusing thing about I2L is it uses multi-collector transistors, while TTL uses multi-emitter transistors. But these are actually the same physical transistor structures, just swapping the collector and emitters for I2L.

Debajyoti, if I'm understanding your original question right, yes I2L does use diffusion for the transistors but it avoids diffusion resistors. It's not diffusion itself that is bad for density, but diffusion resistors.