I still think it might be a small ssd (like 256GB) with a 2TB hdd for main storage. This continues to allow both internal and external expansion.
The main storage is normally compressed in LZ which is not exactly easy to decompress at many GB/s. So they could have an uncompressed ssd storage, with some algo for automatic space management, or an sdk for devs to manage what they need to be copied there. Or otherwise go wild on a very fast fixed function LZ decoder.
Pro or high end models can still have a big ssd and no hdd. And more importantly devs get a guaranteed performance from the ssd, regardless of which drive is replaced or added externally. It allows users to change the internal drive for a 5TB drive inexpensively. (if they have a slot with 15mm clearance this time).