Assuming this comment is made in good faith, it's referring to the fact that it can be delivered in a "flat package", so to speak, and has to be assembled by the customer. Flat being a relative term, rather than a specific width.

This is the relevant wikipedia article: https://en.wikipedia.org/wiki/Ready-to-assemble_furniture

Also see: https://en.wiktionary.org/wiki/flatpack

(First sense.)

It's a fairly common word in English.