Browser performance tips from 2014 mean very little twelve years on. Not only have machines gotten faster and networks gotten faster, rendering engines gotten faster. And I'm doubtful it nested flexboxes would've been all that much of a problem in most cases even then.

The most important thing is to use the right tool for the job. If grid lets you express what you want in the most straightforward way, use it; if flexbox does - even if it needs nesting - then use it instead. Don't shoehorn one into a situation where the other makes more sense. And sometimes either will work for a particular situation and that's fine too; use whatever you find most ergonomic. They're both very good in their own way.