Never mind "physically building". The whole section about the CPU is pretty evasive about who designed it to begin with. They kind of, sort of, imply it's their own design — "our first custom built SoC", "The team that designed our silicon" — but neither statement actually says that it's their own design.
It's their own design, codenamed "Whitechapel". Evidently they got help from Samsung. As for who will physically make it, either Samsung or TSMC, no one else can fab this 5nm design.
Looks like getting Qualcomm out of the picture is one motivation.