It was another carriers LTE device.
Cant remember exactly what brand phone but I think it was a Sprint LTE phone trying to use it on verizon.
Your verizon ipad sim will work fine on a verizon iphone 5 and vice versa since they're both from the same company.
Its just when you're trying to bring in devices from other carriers that they will drive you nuts.
From your links FAQ:
11.
Can I use my Verizon Wireless 4G SIM card in a 4G LTE device not purchased from Verizon Wireless?
Yes. You may activate any 4G LTE device that has been certified by Verizon Wireless to be compatible with our 4G LTE network, including devices not purchased directly from Verizon Wireless. Simply activate your 4G SIM card on a Verizon Wireless price plan.
The bold thing is "certified by verizon wireless"
Most things not purchased under verizon will not get "certified" or approved to be used on their network