It looks like the Comcast agents are a bit green for this type of question :-)
It is so easy to copy/past what is written on the website...
My answer is yes. But you need to make sure your iPhone 7 Plus was either bought throught Verizon or as a SIM free model on the Apple Store.
Apple is selling 2 different iPhones 7. One with an Intel modem and one with a Qualcomm modem. Only iPhone 7 with a Qualcomm radio will be compatible to be used on Xfinity mobile.
But do not buy the LG X. If you buy the LG X, your line will be recorded as an Android line and you won't be able to access Visual Voicemail.
What I would suggest is to order the cheapest iPhone. In that case the SE. You activate the Sim received on the SE. Once the activation is done and all the services like 4G LTE and visual voicemail are working you just run off you phone.
Then you put the SIM in your iPhone 7 Plus. The first thing you have to do is to go under settings/General/About. You wait couple of seconds to see if there are any carrier update. If none you should see under network: Xfinity Mobile.
Now everything should work. The Sim and the phone number are recorded as an iOS device on the Xfinity/Verizon server.
You need to call in to request an unlock code. During that time you need to leave the SIM in the SE. Do not forget to ask for the unlock ticket number. After 24Hrs you should receive a phone call from T2 to confirm that the request has been submitted. Apple should unlock the iPhone SE IMEI within 48-72Hrs.