The reality is, when a CDMA carrier deploys LTE without deploying UMTS, there are usually compatibility layers between CDMA and LTE such as CSFB and eHRPD for when VoLTE is absent. CDMA was never designed to interoperate with LTE as LTE was built around IMEIs and SIM cards but CDMA was built around burned-in ESNs and PRLs, so LTE support was hacked on.
This is why Sprint and Verizon used whitelists: they literally couldn't accept GSM-only devices because you wouldn't be able to make a phone call (the phone would try UMTS but only the non-supported CDMA2000 can be used to actually call, so in turn no phone calls).
Sprint went further by using the CDMA provisioning system on top of LTE instead of just using SIM cards and 3GPP provisioning like most GSM and CDMA carriers. This was a nightmare for custom ROM users like me as custom ROMs were designed for GSM carriers in mind and Sprint was at best an afterthought.
Meh, I was a smart-alec kid there with my friend. The salesperson had to go get a sim card "for the LTE to work" and I said "oh right, because LTE is GSM and requires a sim". The salesman insisted Sprint didn't use GSM so I looked up the wikipedia page for LTE on one of their demo phones and started reading out loud "In telecommunications, long-term evolution (LTE) is a standard for wireless broadband communication for mobile devices and data terminals, based on the GSM/EDGE and UMTS/HSPA standards."
How so? Sprint operated 4G LTE, which is a GSM technology (or alternatively, unified the two, depending on how you look at it).