> how can a single person affect negatively FOSS so much?

FOSS is not affected negatively.

First because FOSS is not en entity in the first place. There is no "community" per se but a muriad of softwares choosing open source licences for various reasons.

Second because the choices of one project maintainer is negligible.

> This type of behavior should be moderated and banned from any FOSS activity

Banned by who and where?

> I've never seen a open source project page without a github link or a git clone ref

Now I understand. You must be new. How old are you? 10? FOSS has existed way before github and will probably still exist when github will be forgotten. There are still many open source projects not using github, some do not even use git.

> what's the advantage of having this behavior?

Not having to deal with external input, PRs, bug reports seems to be a good example.

> I see it only damaging towards the person

Which person? The maintainer? I highly doubt so.

> the project

The project will be fine as long as the maintainer wants it to be active. This is the only thing that counts.

> and the community.

Which community? Other devs? They can (and do) fork when they wish to do so.