Same thoughts here. The comments in here make it sounds like a good idea. But isn't the whole idea behind excel (and any spreadsheet really) to have all data flat into sheets. For me the idea alone seems to break the whole concept
But Excel is not a relational database: for simple cases, having an extra table for a non-scalar column is awfully confusing to present to an end-user. From that angle this makes total sense. There's still 1NF for more complex embeddings, anyhow