Where does it say it’s 120GB NAND inside HDD? Both Amazon and WD say just “dual” drive of 1TB plus 120GB SSD. That sounds like two drives, physically separate.
Which of the two do you think makes sense?
Making HDD faster by adding higher capacity NAND chips onto its internal board
Making SSD cheaper by replacing previous-generation NAND chips with latest-generation ones
The first option makes it faster by 5%. There are some noticeable access time improvements, but you can do better by increasing the size of the DDR-SDRAM on motherboard and using some of the RAM space as cache for HDD. The NAND cache does not make the platters run any faster. The bytes are read and written the same way as pure HDD. The fact that it is so easy to make SSHD look so much faster than HDD actually proves conventional HDD is outdated, ancient, and should been abandoned at least one full decade ago. You pay about 2x as much to gain 5% performance improvement.
The second option by comparison can potentially make it cheaper by 500%, or more by around the time 4TB SSD becomes ubiquitous. But it also makes it faster by 500%.
What I mean is, it’s sane to focus on SSD because it’s easier and cheaper to make 4TB SSD sell at 4TB HDD price than to make 4TB HDD perform at 4TB SSD speed.