Some software developers prefer to host their own repos and have more control over the release process and/or don’t want to fill all the criteria for being included on f-droid, so they create their own repos. Some of these apps can still be found on vanilla fdroid, but often aren’t updated so frequently.
Izzyondroid, on the other hand, is a different project, aimed at hosting different apps that are usually from smaller devs and can’t be included on fdroid yet, for different reasons.
The greatest thing about fdroid is that it allows anyone to create their own repos and you aren’t forced to depend on anyone.
Nice article! Are you the author? If so, I’d like to give you a suggestion: the part about the deepin Linux story and technical background is interesting enough to be its own article. That way, it would be more findable on web searchers and probably help a lot of people who look for information on it, which is a bit scarce in english.