Online/Scaleway is a subsidiary of Iliad, a French telecom operator. Rumor has it (and it's not far-fetched) that they actively encourage heavy upload usage from Scaleway customers to help balance out their peering and/or transit connections to negotiate better deals.

I agree! If at any point they would've asked me to lower my usage I would've gladly done so. But they never did that so kudos to them. :) Especially considering it costs me 3EU a month.

I run one on hetzner cloud that does similar traffic and it is no problem at all for them. I basically max out the bandwidth at any given moment and they never complained.

Why? It doesn't impact them meaningfully. The instances only have a 200Mbit interface so it's not like you can saturate their pipes.

It's one of their many strong selling points - unmetered traffic, even on their lowest-tier offers.