That's literally what it is. The only difference is that all the people trying to download macOS and other updates aren't being malicious and their devices weren't compromised and turned into bots.I don't think you know what ddos is. It's not just stressing a server.
What is a DDoS attack?
A distributed denial-of-service (DDoS) attack is a malicious attempt to disrupt the normal traffic of a targeted server, service or network by overwhelming the target or its surrounding infrastructure with a flood of Internet traffic.
How does a DDoS attack work?
When a victim’s server or network is targeted by the botnet, each bot sends requests to the target’s IP address, potentially causing the server or network to become overwhelmed, resulting in a denial-of-service to normal traffic.