On their site: "will work with 2G/3G/4G, GSM, UMTS, and LTE"
If it's using something like this: http://a.co/9IVPmkI -- then maybe it's using the USB pins on a mini PCIe card.
I think it would be wise to wait until they have committed to a particular cell modem. Most people will not be able to use a phone day to day if it suffers from:
• Dropped calls
• Bad call quality
• Low speed data (anything less than 4G/LTE)
Every mini-PCIe cellular modem I've seen interfaces via USB and not via native PCIe. You can even buy adapters to use a mini-PCIe modem as a USB modem. [0] I don't think this is standardized, but it seems to be the defacto standard for all modems in this form factor.
Now this may change once LTE/5G speeds regularly exceed what USB 2.0 is capable of since mini-PCIe does not provide pins for USB 3.0, the standard is too old. I would guess they'll move to NGFF as that's already the case for newer business notebooks which offer a WWAN slot.
[0] https://www.amazon.com/PCI-E-Adapter-Module-Support-connecto...