You host it on your own Cloudflare account.

Thats not self hosting, is it?

Most people consider self-hosting as "running service x yourself", which I think i fair considering most people treat self-hosting as an alternative to SaaS offers

Cloudflare is a SaaS.

No, Cloudflare is a company that offers many products. And the services utilized by the project (Pages, D1, R2) would most likely be classified as PaaS by most.

I think the novel hack here is taking something typically hosted on IaaS or lower (whether that's a VPS, colo, under your desk at home, etc.) and instead running it on PaaS.

It's a hosting service where you host your own copy of the code. It's no different than getting a VPS and doing the same thing.

If you have no access to the code then it would qualify as being hosted for you.

Are you self-hosting your pictures when you upload photos to your Google Photos account?

No. Self-hosting refers to running a service where you completely own the source code. You can run this on a computer in your room, or a rented VPS, or a serverless hosting option like Cloudflare Pages.

For example, if you run a Nextcloud instance and upload pictures there, then you are self-hosting your photos.

Self-hosting means the ability to host and run the code on your own server (home or webhost). Since Microfeed is built on the cloud services offered by Clouflare, you technically can't run it independently on your own server as you will still have to use the Clouflare services:

    microfeed uses Cloudflare Pages to host and run the code, R2 to host and serve media files, D1 to store metadata, and Zero Trust to provide logins to the admin dashboard. 

@jacooper is correct that the term has been wrongly used by the creator.