First, Signature Edition PCs are just Windows 10 systems that don't come with vendor bloatware installed. There's nothing here that blocks alternative operating systems. The "product expert" is probably a tech support drone monitoring Best Buy forums.
The real reason Linux won't work with the 900 ISK 2 -- or with the Yoga 900S and Yoga 710S -- is Lenovo's proprietary SSD driver. This is not a Microsoft product. Indeed, Windows 10 won't run on these laptops either without the additional driver.
Lenovo has not provided this driver for any other operating system. It also seems to have no plans to do so.
The company told me:
To improve performance, the industry is moving to RAID on the SSDs and Lenovo is leading with this change. Lenovo does not block customers using other operating systems on its devices but relies on the alternative operating system vendors to release appropriate drivers. When the driver is released by Linux, users will be able to install the operating system on an SSD with RAID.
Ah, excuse me, Lenovo is the vendor in this case. There is no Linux company that supplies drivers. There never has been.
Instead, vendors may either provide their own drivers, release driver code for the open-source community, or provide hardware and engineering details to the Linux driver kernel community so that independent developers can create drivers. In this case, Lenovo isn't doing any of these things.
Historically, Lenovo has done well at supporting Linux. This time, it's fallen down on the job. Microsoft, which used to be Linux's enemy No. 1, is innocent here. I know for many Linux supporters it's hard to believe, but Microsoft has become a leading Linux supporter.