Mix of elements in block level elements

This is from the section of the course “Inline formatting context & anonymous boxes” from the module “Unknown Fundamentals: formatting context.

“When we have a block level element, it can either contain other block level elements, or inline elements. It cannot contain a mix of them.

But from what I have been learning, it appears a block level element, (such as a div), can contain both other block level elements & inline elements simultaneously. Is anyone here familiar with this?
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