If you look at the form factor, the hybrid drive is most likely a 2.5 inch drive. As far as I know there are no hybrid drives in the compact ssd size (either m.2 or msata format - the spinning platter wouldn't even fit). Just by listing that it uses a hybrid drive, they are already revealing what the form factor is - which is larger than a m.2 slot, and therefore competes head to head with the last gen 2.5 inch SSD with the SATA connector - which at 1TB is around 300 bucks these days.
And yes the SSD's are not as fast as the latest m.2 drives, but are still miles ahead of any spinning drive. I'm not really sure what you are trying to say ... like a Ferrari is faster than a Tesla in Ludicrous mode, so ... go for an electric hybrid like a Toyota Prius? I know that the car analogy doesn't always work, but can you think about your reasoning? That a true M.2 SSD (which can be thousands of dollars) is faster than a last gen 1TB SSD in the 2.5 inch form factor, so therefore MS should use a spinning hybrid drive???
You can wait for a teardown, or you can use logic and common sense.
The only other choice is for MS to use a non standard size spinning platter, maybe around the old Compact Flash drive size - but in that case, there is absolutely no point - they should have just gone straight to M.2, especially for something in that form factor.
To be honest, current generation hybrid drives pair up a really slow 5400 rpm drive with some flash. Now it can still get good performance, but in the end, you're dealing with a hard drive from a few generations ago. Seagate had 7200rpm hybrid drives last generation, but they recently changed to 5400 rpm drives, which makes the drive kinda slow, even with the flash addition.
Microsoft may certainly have gone custom with custom parts, but I don't think they have the volume, even in the wildest expectations, to go against the now popular ssd juggernaunt that is overtaking all of the slim laptop (Apple, Lenovo, Dell, Razer, etc), small form factor embedded pc, and high performance pc space - for good reason. It just makes sense, the parts are really widely available, and people are expecting that type of performance.