TLDR: I don't think her examples are any good, so this clip gives the "rationale" behind it but no actual-world examples to really understand the differences.
I don't think the example she uses is a good one. That thing is obviously achievable using absolute positioning, which in this case is arguably easier and will give you much better browser support.
Also, the other examples she uses are really weird. Boxes of varying widths and heights. This is just not a thing in current websites.
The flexbox example where she ends up with 3 items on the first row and 4 on the second is just fantasy according to my experience. That would only work for a manually crafted page, whereas most websites with that much content items are auto-generated from some backend (even if it is markdown files in a static website generator). Otherwise please show me how can I achieve that effect with similarly sized items and how do I make it responsive.
