It is an infringement. However, unless a patent infringement notification is served on Amazon and the mockup company (or less plausibly the buyers), the infringement would not provide for a basis of a suit unless the fact is established by the of-jurisdiction court that the infringer knew about the invention, and willfully infringed. (see also the question: "Can you safely do anything without infringing patents?")
Although the mockup company may obtain a patent on the improvement if filed for, and found non-obvious, but that would not protect them from the infringement of the cited U.S. patent (#10,155,479 B2). It would merely mean that if the manufacturer of the original also wanted to start a foldable one, now they would need to deal with getting a license from the mockup company if they would have obtained a patent on the foldable invention.