It's a strange claim - to say it's a terrible idea. I've been programming for over 20 years and I've been using a font with ligatures for nearly a year now. For me, personally, it increases legibility and clarity. I feel slightly stunted when I don't have them. I read other people's code with ligatures on just fine as well.

I also hate dark mode and small fonts, and I let my code run way past 120 chars per line when it's right to do so.

I couldn't possibly care how other developers read code. I want them to be able to read my code and so I try to code for clarity, but I can't imagine a font choice or many other IDE preferences affecting their ability to read the code I write.