He said on any triband phone( a phone that supports band 25, 26 and 41). The m7 is only a single band phone, it only supports one lte band which is band 25. All triband devices so far only support one transmission path, which means it can only connect to one at a time, so when you get a call it disconnects from lte to go to 1x and when you finish the call it disconnects from 1x and reconnects to lte, never both at the same time which is what you see on signal check. The m7 on the other hand supports two transmission paths thats why you can be on a call and still connect to lte at the same time, thats not the case with a triband device like the m8, g2 and gs5.
Edit:
(the blind leading the blind)
This is another reason why there are so many complaints when dealing with customer service, a lot of this stuff just don't make sense to the average consumer and to make it even worse a lot of this same stuff don't make sense to your average customer service rep either. So unless you get with someone that really knows their stuff(which is rare to be honest) they would just exchange the phone out for you not realizing that there isn't anything wrong with the device at all because they don't know how to effectively communicate with the consumer so they could better understand what is actually happening.