There are various ways for them to mask the traffic, but also even if you have a list of a million IP addresses, what does that actually get you? Usually, it's millions of compromised devices regular people own that are being used to initiate the attack. IoT botnets are a kind of scourge. SE could be being attacked by TVs, smart light bulbs, security cameras all controlled by malicious actors. Botnets can be very large and even if you block the first hundred thousand devices, they can send a different hundred thousand devices at it. It's a tough problem to solve.