I don't think it's a botnet. To me that implies that the software that runs on your computer is a service that talks to a C&C server or to other bots P2P, forming the net(work) part of a botnet. In this case it is not a bot downloading a payload from a C&C, but a user intentionally visiting a website which downloads and runs the payload. It does not really work as a botnet since the web browser is not a service that will continually talk with a network of other servers to get a new payload.

Try ignoring what the payload is as it does not really matter in defining what is or isn't a botnet (though botnets typically imply malicious or sketchy payloads).

In regards to it being a DDoS it leans more in the yes direction due to the intent of it, but it also sites sending traffic to other sites is part of the web. You can embed things from other sites like images and then those others sites will fulfill those requests. The web didn't restrict pages to only send requests and load content from the same domain.