It's particularly jarring to basically every site I've seen it on which is usually some serious and professional looking open source site.

I wonder why nobody configures this, is this not something that they can configure themselves to a more relevant image, like the GCC logo or something?

Because that's the difference between the paid and free versions

Anubis asks that you don’t change the logo and if you want to, pay them: https://anubis.techaro.lol/docs/funding/

I think they might also want to bring attention to the problem and advertise for an open-source solution.

Fire up your LLM of choice and make a web extension to make it more presentable. Remove the logo, generate one, do whatever you want. The world is your playground, don’t let it “jarr” you with stuff.

That's the paid upgrade for "enterprise" level quality.

I’m sure if you want you can offer to pay like $500/mo on their behalf and they’ll change it for everyone.