As described, they weren't taking home devkits. The XSX needed to go home in a tower PC/Subwoofer. Lockhart in One S/X, which is one reason the the Series S is assumed to be flat and smaller than the One.
The point is the XSX is supported by a flat form factor. There is nothing to say XSS can’t support a flat and cube format. If we are talking about “cases” meant to disguise the hardware and protect the nature of the form factor then a flat case would suffice.
However, I’m not sure if the info given is referring to boxes and not cases because I’m doubtful that MS would require employees to roll their own case setups to take the Lockhart home. It’s not like Xbox hardware or form factors are plug and play in nature like a PC. If the staff were asked to use a PC/subwoofer/XOS/XOX boxes to disguise the units then it would make sense that that would indicate the shape of the form factor.