It's not, FlexDiv is just another name for Stack, they are describing the same aspect. Eg. you can imagine one CSS library using Flex, one Group and the third one Stack for the same concept, eg. some other UI frameworks use VBox and HBox. But it is still different from TopSectionHeader, SectionHeaderInner, SectionBodyWrapper, or whatever other names you have to come up with to give your component "semantic" meaning according to their content.

Because flex just references the layout engine, and div intentionally has no semantic meaning. Fwiw I don't think InlineStack is a particularly good name either, but the criticism of this needs more justification than just "it's not how we did things in the past".

Semantics evolve over time. This is true of natural language and coding practice.

I instead use <div class=“flex”><\div> now.

If the class name is not enough for me to guess, I just hover the class name and the LSP will show me the applied styles.

Another issue is that Shopify changes the name of these components for no reason, for example InlineStack was once called Inline, then HorizontalStack, then InlineStack, while the flex box model hasn’t change since 2012