The metaphor only works if you know what the object was.

A portmanteau — French porter, to carry, plus manteau, cloak — was a travelling case that opened into two equal compartments. Not a bag. A case with a hinge down the middle and two halves, each packed separately, closing into one object you carry by a single handle.

Which is exactly the claim Lewis Carroll was making in 1871, in Through the Looking-Glass, when Humpty Dumpty explains slithy to Alice: it means lithe and slimy, and there are two meanings packed up into one word.

Two compartments. One handle.

The distinction that gets lost is the one Carroll's metaphor actually encodes. A compound joins two whole words — bookshop is book plus shop, both intact, separable again with a pencil. A portmanteau fuses them and loses material: breakfast plus lunch gives brunch, and neither word survives. Smoke and fog give smog. Motor and hotel give motel.

The loss is the defining feature. If you can pull the two words back out unharmed, you have a compound. If letters went missing in the joining, you have a portmanteau — the two meanings were packed, not stacked.

One small footnote about who owns the word. Linguists generally say blend. Portmanteau word is Carroll's literary coinage, and it entered technical vocabulary through the back door and then outlived the technical term in ordinary use. Almost nobody outside linguistics says blend; almost everybody says portmanteau, usually without any idea what a portmanteau was.

Which is a fittingly Carrollian outcome: the word for words made of other words is itself a French compound, describing a piece of Victorian luggage nobody has owned for a century.