I hate the "drunk after one beer" trope, because no one gets like that after one beer, but hey ho.
You can't be serious. I've known several people who get drunk that easily, including my first college roommate.
Aaaand it’s an excuse to behave badly as they can later blame it on the booze. It’s all psychosomatic, so pass the placebo!
No, it's not--some people can't metabolize alcohol properly and can get shit-faced, lost-memory drunk on a drink or two. I've known at least three people like that--one who could nurse a beer for hours and be mostly OK except for some slurred speech.
Couple things:
1) As mentioned above, some people truly are weak to alcohol. Had a college buddy who turned an alarming shade of red and needed assistance walking after one Long Island.
2) Tolerance is a thing. When I started drinking, a couple Mike's put me under. Now I guzzle Bell's Two-Hearted.
3) There is this misconception that all beers are weak. They are not. Every brewery I've visited has at least one brew that exceeds 20% ABV. I've personally seen as high as 35% ABV, and a quick inquiry on Google-sama shows that in 2013 the world's strongest beer was 67.5% ABV.
Bottom line, unless you know what beer someone is drinking, don't assume it's a Budweiser (5% ABV).