DOTA player here: Players in League call stutter-stepping orb-walking. That's just how it is. When I was talking about it with my brother (who plays more League) I was annoyed that it's called orb-walking because there are no orb effects in League that can be manipulated like that, but at the same time it's really not a big deal to differentiate them between games. Some games are just going to use one term to mean something else, and that's just how it's going to be.
Now, on to ADCs, as that one of my preferred roles and I tend toward support and pos 1 in DotA2: Yes, every squishy champion needs to position correctly to not die, but unlike most squishy champions ADCs have to do their damage over a sustained period of time. It isn't "Oh, at least I dropped my burst combo and killed a couple people" with most ADCs if they die quickly. For them it's more akin to "Holy shit, I didn't have time to do any damage." even if they both die within, say, 2 seconds. Unlike most other squishies, staying alive is paramount to do anything at all, not just live. A mage? They can do their combo and if they die then at least they did the bulk of their damage. An assassin? If they got the enemy ADC or mage then it was likely a worthwhile trade because most of their damage was going to be on cooldown for a bit anyway. For an ADC? They have to live to pump out their damage because they don't have the burst most other squishies tend to have. This makes them reliable in terms of sustained damage, but unreliable in that if their positioning is bad then you'll never see a bit of that damage come to fruition. This means ADCs, by their nature, are only ever easy, safe and reliable if the enemy team is basically worthless.
On to tanks: A tank is something that demands attention because of the disruption they cause, not just a damage sponge--this isn't WoW, there is more to being a tank than not dying quickly. They disrupt. Mundo doesn't disrupt through anything but damage. Tanks in video games, unlike real life, also don't tend to hit very hard, and so Mundo, who has high damage and survivability but low CC is not really a good tank. He's good at being a bruiser, but if he needs to protect an ally (something tanks are known for) or go into an enemy team and disable an important target (alternative thing tanks are known for) then he simply can't do those. His option is to run into the enemy line and hope to god that's good enough to draw meaningful enemy fire with the threat of his damage. Darius has pretty much this same problem in classifying him as a tank: He's hard to kill, but his main source of disruption of disruption is damage. Like Mundo, he has lacking mobility, low CC, but he makes up for that by bringing high damage and high survivability, I.E. not a tank.
and etc, these are tanks. They can, in fact, CC key targets or disrupt an enemy line on demand, they can peel for allies, and they can take a beating. Damage levels are variable but it tends to meander somewhere around "low" to "mediocre" when properly balanced. They do enough damage to threaten a squishy, but their damage isn't particularly intimidating for someone building defensively, as opposed to Mundo and Darius who can threaten someone building defensively. They give you reason to focus them other than just damage; damage alone only puts you so high on the threat meter, because damage alone can't guarantee a kill. CC in a teamplay scenario, on the other hand, can net kills by making the damage significantly more targeted in that area or player. Like the ADC, the tank would like to live as long as possible but not for the same goal: he does it so the enemy team can't just get straight to killing squishies easily as long as a CC machine lives and threatens their livelihood with threat of being put in a very compromised position. Darius and Mundo want to live longer just to put out more damage because that is wherein their value lies. They are not tanks. They will never be tanks. They are juggernauts.