Overall i'm pretty sure that "list about 20 differences" is relevant in some cases/procucts/websites, but not for gitlab - they just proved that by disallowing working on Windows (which wouldn't disallow testing on a VM).
But here's the important question: are the problems that are caused by ignoring the aforementioned differences enough to prevent people from being able to use your software? Or, conversely, are they enough to weigh more heavily than the functionality that you provide for your users?
Because somehow even Electron based software is pretty popular in the desktop, oftentimes looking and feeling rather different from most of the native software. But apart from nitpicks and rightfully complaining about those annoying inconsistencies, does it really matter enough to spend resources towards fixing them?
I can't think of that many inconsistencies that caused web applications to be unusable across different platforms altogether, apart from IE not supporting certain JavaScript functionality back in the day and non-transpiled (to say, ES5) codebases breaking altogether. But look and feel related things have hardly been as important.
The browser is wired into the OS in real ways, and they matter. Everything from "How does the os manage passwords" or "how are system notifications presented" or "How much screen real estate do I have have on a given window size (scrollbars...)" to subtle differences in how the network stack behaves, or how windows are rendered.
Hell, I can list about 20 differences just off the top of my head - and a damn good part of that is because I've been forced to pay attention to each of those platforms. Which clearly you have not...
I read your comment as "I only use a mac, and I don't understand".