No, people who don’t stream aren’t counted in the above statistic (which is trivial to spot check)

Another possibility is just using it to capture your own play, for instance if you want to review matches later to try and analyze flaws in your game.

That's massive stretch. Why would anyone do that? Twitch player isn't that great if you want to seek around the video

One use case is speedrunning. If you don't have space on your PC and want to record your runs.

It works easily and keeping hours and hours of video locally still requires quite some storage space

My OBS recordings (of mostly game footage) come out to between 1 and 1.5 GiB/hour for 1080p resolution. Given that simple external HDDs start at about 20€ for 500 GiB, that comes out to 6 cents of storage per hour recorded. Your power bill for the gaming PC is going to be higher than that in some places.

OK, so we’ve established that it is possible to do it another way if you want. But why get an external hard drive if you’re happy with Twitch VODs?

Twitch VODs are only retained for 14 days (or 60 days if you're subscribed to Twitch Turbo).

Yeah but are you going to want to review years-old footage or stuff from the last few weeks?

I have only streamed for slightly over a year, so I cannot say anything definite about "years-old footage". But I can absolutely see myself going back to that footage when I get nostalgic about any of the games that I've played. Especially with games that you can only realistically play once, like Outer Wilds.

I only have really used it for fighting games and my object was more to look for issues in my game. So there isn’t that much point to looking at very old footage.

I myself have used it for this purpose so I don’t think it’s a massive stretch.

There are 150+ million accounts on twitch, but only 10+ million people who streamed in the last months, and around 5 millions who seem to stream regular. Doesn't make the impression of being skewed by people who did not aim to stream. Sure, some might hit the button by accident. But more likely, it's just people who have a different aim than making viewers.

I think you're making the mistake that the point of a Twitch account is to stream. Accounts are useful for viewers with no intent to ever stream as well