many eu countries have gigabit per second connections, so thats like 2300 infected hosts at least. not many at an internet scale.

Many providers in EU countries offer 1gbps fiber as an option, but the most sold option is more like 200-250mbit because the price is lower. And many of those are cable based which means only 25 or 50mbit uplink, not symmetrical like fiber. So that would require 100,000 endpoints participating.

But this was a reflection attack, so most of the bandwidth was coming from poorly secured servers. In datacenters those would most likely have 1 Gbps uplink speeds.

... Or only 2300 high value endpoints. You can select your endpoints, you know.

Also, even cable is capable of Gigabit speeds. I briefly had 1.2Gbps via cable, downgraded to 400Mbps as it became cheaper.

I feel like you didn't read the comment you replied to carefully.

Did you have GB upload speeds with cable too?

Not gigabit upload, but I got close at 700Mbps, and some cable providers offer gigabit upload speeds in some regions, cogeco for example offers gigabit symmetrical cable in Trois-Rivières.

France is widely offering 1 Gbps symmetric fiber in major cities. The 3 major ISP offer the same unique package for 35e a month with internet and phone and TV, not negotiable. That's gigabit fiber if your home is in a area covered by fiber.

Do the ISPs have enough outgoing bandwidth to the internet for all their 1 Gbps symmetric fiber customers to maintain a 1 Gbps sustained upload simultaneously?

no

But any given provider won't have a 2 Tbps uplink out of their own network. It would likely have to be highly distributed across many providers. ISP's don't build their networks to support all customers running at max at the same time, as most consumers will only push that 1Gbps limit for short bursts.

I don't see why they would not have Tbps uplink.

ISP infrastructure is symmetric fiber. A single fiber can do 100 Gbps without troubles. And when you have to bulldozer the road to lay the fiber, you don't put a single fiber but a pack of a hundred. There is no crosstalk between fibers unlike with copper, so one has interest to put as many as possible.

Sure. You can spread your attack over a few countries and as a result a good 20 ISPs. I assure you if your ISP offers cheap gigabit they have a 100Gbps uplink.

This attack specifically involved servers.