The "code" you linked to is not functional code of any sort. Not to be nitpicky, it's just an HTML image tag, so its Markup at best. All you did was stop the loading of an SVG image. The fact that they source it from their own domain tells you everything: they have a script that runs to check the current number of stars, then generates this image that reflects that. SVG is an image format. It's really standard.
All your other points you're making because you do not have much experience in the software realm, which I'm not saying to be dismissive or anything at all, I'm simply illustrating that all the points you're questioning or mentioning are 100% standard.
you don't make a fork for three lines of code and ask others to "check it out". If anything, just point out the issue and post a diff or a script to fix it. Simple.
They have a pro version, and are using images they generate in a template viewed by users to promote its popularity and try to sell pro. They're running a business out of this. Not every FOSS project is non-profit, and these people are simply trying to sell a product AS WELL as keep it open source for others to enjoy, like yourself. Feel lucky to have the privilege they are letting you use it for free.
The term "phoning home" as you're trying to use it, is wrong. You're implying that it is functionally doing something unexpected. It is not. It is sourcing an image in HTML. The suspicious type of phoning home is code that executes locally and pulls down other functional bits of code that alter the way the software APPEARS to be used. It's a way of obfuscating something shady, like a virus, or malware. This is not that kind of code.
If your concern is simply that the code you've run is sourcing an image from somewhere, I can only imagine how upset you'll be to learn that software repos of this size are pulling things from dozens, if not hundreds of places. This project pulls from rubygems, yarnpkgs, and the dreaded example.com.
Lastly, the reason that team responded to you in that manner was more that they were taken aback. Like "WTF is this person talking about? I don't get it." Realize that they were nice enough to respond, where most project maintainers would just ignore or close the issue.
Also, you might want to freak out about the social badges being sourced in this as well. This isn't a "privacy first" project or anything. They aren't doing anytweird, you're just misunderstanding some things.