I wasn't even going all the way. But fine, I'll go all the way.
Twitch at 18 has 96.5 AD. 40 for ult, 40 for GA, 70 for IE, 50 for LW upgrade. That's 321.5 with runes and masteries added in.
321.5 x 2.5 = 804 damage. 804 x 1.05 (masteries) x 1.2 (LDR) = 1013. 2% of that is 20.
321.5 (AD x 2.5 x .4) + 20 (feast) +20 (2% lifesteal) + 75 ( x3 ardent from runaan's)= 437 x 1.08 (runic armor) = 472
That's about 25% of Twitch's HP at 18. You do have armor to factor into the 20 from lifesteal, but there's also 3 targets from runaan's, so we'll keep it at 20, since its such a variable circumstance.
That's getting pretty close to 30%.
I very highly doubt he actually measured how much he saw. He likely saw a twitch at 1/4 HP or something and saw him heal a huge chunk of his HP.
That's pretty damn close to his estimation. He was probably heavily exaggerating, but a situation VERY close to what he claims is actually possible without any purchased lifesteal.