From CSS 2.1,

A block container box either contains only block-level boxes or establishes an inline formatting context and thus contains only inline-level boxes.

That disappeared from CSS Display. It defines block container as

A box whose contents participate in a block formatting context.

But if the box container only contains inline-level boxes, the box container will establish an inline formatting context, and the inline-level boxes will then participate in an inline formatting context, not in a block formatting context. In fact, that's what inline-level boxes do by definition, they participate in an inline formatting context.