I think they mean it’s a complicated solution for an uncomplicated problem.

Grid systems existed long before CSS grid and were dead-easy to understand and use (960, Bootstrap etc). CSS grid, by comparison, is completely unintuitive. Sure, it’s definitely more powerful, but the barrier to entry is way higher.

Exactly. "Align things to a grid" seems like a simple concept, but CSS grid seemingly has at least 3 ways to place the top-left corner of an element, and it just gets worse from there. I tried it for a simple layout with 3 elements and quickly decided I prefer flexbox, tables, or even floats.

I think when you come across situations that only grid can solve, then you appreciate it a bit more.

Flexbox complements grid imo, it's not an alternative.

That's fair. I only use CSS for personal/hobby projects.