Sejuani Passive - Am i missing something very important?

First I would like to say I love tanks - I play tanks, and bruisers, and all things tank - but even as a tank player, who hated when Bork/Lord Dom's + Vayne and Vel Koz ripped me open and danced in my proverbial guts. I have to admit the Sej since the update seems - WAY tankier then she should be , even from level 1..

Now, not wrong as in she gains the armor/Mr, its a great passive, its cool, its useful and its decent.. I just think it needs to be lowered and suited to scale with level or something - but that is another matter -

I will give a example.. In a game I was recently part of

Sej has

vs Hemier with

Now, There is a team fight.. all for fun and fun for all, So Sej passive drops after the first few seconds - Heimer drops Mega Turret , and 2 small Turrets, and hit Sej with a Grenade Stun - and followed with rockets - running passive death-fire ..

Sej - ignored him and proceeded to just suck of all the damage from him as well as the team fight

now i will give it that perhaps the turrets took the first few shots at everyone - but when all is said and done stat wise she had 53.4 MR (Level 18 base stat) + 65 (Abyssal) (her only MR item) and I think the runes give about 20, so lets say that, since obviously no idea what she was running in runes, 20 might be good or bad, so total is 138.5 give or take ( from the information i currently have)

now to the Yordle Mechanic Void staff - 40% Pen - Torment - 15 Pen, Sorc Boots -15 pen - and 12.5 pen in runes - total of 40% + 42.5 Flat Pen. Now i do not know exactly how M-pen works, prehaps one of you can help, is it .. ( MR - % = X - Flat = Adjusted rate? or MR- Flat = X - % = adjusted rate?) So for the sake of fun i listed both.

40% of 138 is 55.2 taking the MR to 82.8 (138 - 55.2=82.8) - then taking Flat Pen into account, 82.8-42.5 = a total of 40.3 MR left that can mitigate Magic Damage - or 138 - (Flat)42.5 = 95.5 - 40% - (38.2) = 57.3 - (this is the way I "think" it works)

All and all the point is that there is really no way she should have been able to just chill in 3 turrets- with the Torment Burn, Deathfire Burn - and the shots from the Mega + small turrets, the way she did.. is there something I am missing? I love that she is upgraded, but that seemed a bit "over the top" even for a guy who loves tanks.

I dont care who won or lost, I just want to understand better the mechanics of the game. Thanks for the help -