EE (formerly Everything Everywhere) is a British mobile network operator, internet service provider and a brand of BT Consumer, a division of BT Group. Supposedly the #1 network in the UK similar to Verizon in the US.
This is odd advice, when you consider many kids in the same age group probably have access to (or own) a tablet device of some sort. The only difference with a smartphone is the ability to call and text, and portability while staying connected (assuming many tablets aren't 4G/5G capable).
A tablet is a device you give your child to use for a limited time, ideally supervised.
A phone is a device they carry around all day and also use while you're not there.
I won't give my child a tablet before they're 11 either. I'd rather give them a desktop PC and a drawing tablet.
But a phone is now pretty much necessary starting with secondary school, unless you want your child to be bullied and socially isolated.
I heard a podcast with the author of this book and the conclusion was similar. He recommends no smartphone before 16. Dumb phones for simple communication can be whenever.
I haven't read the book yet, but the podcast discussion was fairly informative. I think it was Hidden Brain's Escaping the Matrix episode.