I've called my wife a couple of times, and my father once and did searches while having them on speaker. Worked with no problem. As to technically why it works, I have no clue.
You're the first person I've seen who has reported that simultaneous voice and data is working on Verizon with their Nexus 6. I wonder if there's been some sort of very recent update that has enabled VoLTE for Verizon given that they are supposedly about the launch the phone in stores. That's the only way it could possibly work that I can think of given the hardware in the phone.