Are we also forgetting that Twitch is an assassin too?
Here's what happened during the fight:
Leblanc used her E just a bit earlier in the skirmish (and missed)
Twitch snuck up on her with his q and used his ulti (giving him both a high attack speed and ad steroid) while also having botrk and its active) and got off 3 auto attacks before LB hit him back
Leblanc blew her load with q ult w and gunblade active so her passive wasn't proc'd
LB ran away while Twitch shot her in the back and killed her with his E
Twitch made a play on LB only having 3/4 of her kit up while getting the jump on her in the fight
LB then proceeds to not proc her passive
This is mostly a misplay by LB if anything
If she had her E up or even just waited for her passive then Twitch, in all likelihood, would have died. Instead, LB did not play to her kit strengths and then tried running away after realizing her mistake. Twitch then proceeded to lay waste to the retreating Leblanc before executing her with Expunge. Lifesteal didn't make any real difference in this fight either as Twitch only was able to get health back during the course of LB's retreat (was full health> LB got some burst> twitch auto attacked and got like 100-130 health back while taking no return fire)
Twitch killed LB here with 8 autos (7 were on the real lb and one on the clone) and killed her with a full 6 stack expunge
Edit: The next part is even better
Twitch is up on LB in items
Twitch 3 complete items (botrk, runaan's, and IE) to Leblanc's 2.5 items (gunblade, morello's, and nlr with a blasting wand)
Leblanc has no gunblade active, no ult, and 80% health
Leblanc uses her W to get over the wall and actually FLASHES on top of the full raptor camp
Twitch uses botrk active and pops ulti
Leblanc hits her E on a raptor and hits Twitch with ONLY her Q
Twitch gets 2 autos off which both crit
Shen uses his ult on LB because she just got chunked for about 800 damage from Spray and Pray Infinity Edge crits to the face
LB has nothing left to use and Twitch fires another auto at her then uses Expunge
Twitch fires another crit empowered auto at Leblanc and cancels his last auto as LB just now gets and uses her ult
Kennen and Kha'Zix are coming through the bush
Twitch Flashes away
Fnatic disengage
Seriously, when LB actually engaged Twitch she had no gunblade, ult, or W as she used to get over the wall instead of flash. So in the end she had 2 abilities to kill (or even chunk) the rat with and she failed to land the most important one by hitting a raptor. #LCSbigplays
If LB was full health at this point Twitch would have needed another 1-3 autos depending on luck from rng (3/4 of his autos were crits with 50% chance at this time)