The author doesn't say you can't use features with 98% (or even less) support.

What they say is that you have to ensure that your site still works for the remaining users, through graceful degradation.

If people have new fancy browsers, use their features to make the interface jazzy. If they don't, ensure that the site still offers its core functionality to them without the fancy features.

That’s not actually possible with the example given in the article (dropping scss and moving to nested css), it’s all or nothing for many cases.