He has a point on the Assassins, literally EVERY Assassin is taking Duskblade to get the Lethality and it's bonus to killing wards/shrooms. Adding in the sweeper is the easiest thing ANYONE does to counter a Teemo. As for the other 2 items, both are put in many tank builds for various reasons. Basically a Tank gets so much HP and HP Regen and can run over all the shrooms. The nerf to Liandry's Torment to make it's only 1% Max HP per Second for 3 Seconds equaling to 3% of their HP normal and 6% when they are slowed made Teemo's Shrooms pretty much something you just watch for when low on HP as it'll only be about 350-450 normal dmg + 3%(6%slowed) damage. Now if you've gone full AP that base number might be a bit higher. However when you compare to the ADC's Tank buster item, Blade of the Ruined King, you have 8% of their Max HP taken away per Auto Attack, which for many is 1.5-2.1 times per second. So an ADC can chunk a Tank down where an AP based attacker, or mage, like Teemo can only hope their spell does enough to kill them in the first place without LT's % HP dmg.
Also adding in the Guinsoo's Rageblade DOUBLES that on hit from BORK but still considers LT's damage as spell hit, you really have a massive advantage to any and all ADCs vs a Mage, let alone a Teemo. Now granted I have been a Teemo main myself for a long time, I do ok with him but it's often cause I garner enough hate that my other teammates have time to push objectives while the other team is trying to catch me. All in all Teemo has a hard time fitting into a team as many see him as a weak champion who is easily countered by most Tanks and Assassins right now. His Range of Auto Attack is lower than MOST MAGES, so going mid is hard, and going Bot Lane is next to causing Inting by your team as they bitch at you more than anything with how your Auto Attack range is FAR lower than ALL ADCs. Top is the best place to go for him or Support if you want to be the bully support but you risk the ADC getting free shots on you at all times. As for his Top lane, it's fair but there's way too many champions who can counter you with how your range and kit are currently setup, your best hope is to turret hug and hope for a mistake on their part. Also with the current TANK meta, you have EVERY team you get on bitching for a Tank Top, Jg Tank, and Support Tank so you have the steam roll game.
Giving the Tank buster items a bit of love would fix this as they could then be used by more people, however with how Crit works right now it's also something you don't see ADCs get as they can often get the same or better damage out of having an IE and enough crit to get over 50%, which is just 1 more item with IE. Teemo in general could use some retooling as his kit features him as someone who can counter an ADC but with so low of a range that he's going to likely be unable to do so without a lot of free poke in before he gets in range.
congragulations you get to run around teemo and do whatever. Dont even fucking get me started on adaptive helmet you legit get to ignore teemo because he has nothing with burst to kill you dont even get me fucking started on the ways to kill his shrooms to 
legit assasins buy it everymatch so one assasin his shrooms cant be used because there onhit i mean its like late game is shrooms are so bad there better of waveclearing.
