That is not at all true. A single fiber cannot handle the world's internet bandwidth. And the PON systems used for homes don't even dedicate 1Gbit to each termination (house). You don't have a dedicated connection to a chassis with 2,000 other customers, you are PON split from a single fiber with a lot of other houses, then that goes to a chassis.
"It doesn't matter how it is shared as long as there is no congestion." is a useless truism. It's true for copper too.
I think it's hilarious that you think that your ISP is only oversubscribing their links 2x (2,000 1Gb connections to 1Tb backhaul). That's fantasyland at the prices that residential customers pay.