The word "download" is used in two senses. The first is the broader sense you're referring to, where it means "to receive data." The second sense means to collect all the data from a particular file or dataset and store it locally, as opposed to "streaming." That second sense is the one clearly being used when referring to "YouTube downloaders."

If you view the entire file they are the same thing. And they're always being collected and stored locally even if it's hidden from you.

And a static mp4 file encoded with ffmpeg -movflags +faststart -start_at_zero can be seeked within in any browser from any static webserver and viewed at any seeking time without downloading the rest. Is this streaming too?

Given the first example of fully watching and the second implementation of streaming with a simple static file you can see the streaming/file download distinction you make is much thinner than it first appears. At least from a technical perspective.

In lawyer-ese you are making a valid semantic point which I do acknowledge. "Streaming" in lawyer-ese means a corporation is doing it so there's: 1. legal liability 2. lots of abstraction layers involved so they don't get upset and 3. the local files are hidden from view of the user. Unlike the simple streaming example with a properly encoded .mp4 file.