I'm not against using font style; I'm against (or more like, failed to see the point) to manually craft a complex "system font stack" to achieve.. basically the same thing as sans-serif but worse.

If you're building a web app (say, via electron) as opposed to a site or document, using the system font stack will make that app fit into the desktop environment much better than Arial, Times New Roman and Courier New will.

But that's not what was being compared.

How would electron apps render a "sans-serif" font?

What is being compared? Because to me it read like a question of choosing 'sans-serif' over 'ui-system-font-sans' or whatever in your CSS. Nothing more than that.

"System font stack" — which I took to mean the longish list of commonly known default fonts for a bunch of OSes brought up earlier in the thread — with a CSS value of "sans-serif".

There was no mention of direct use of Arial, Times New Roman or Courier that I could see, so I don't see why bring it up.

He/she is trying to say what the default of "sans-serif" would be if not 'ui-system-font-sans'.

And it indeed is the same font on Android and iOS, at least. Windows is kinda tricky though.