she was told they advertise device limits as they don't want people signing up to the scheme as it's costing them too much.
That's absolute nonsense. I really don't know why people make up stuff like that.
They advertise the average/rough number of devices each package speed should be good for.
They advertise the same average number of devices for the essentials package as they do for the equivalent fully priced package.
They advertise a higher number of devices for the faster essentials package, which again matches the equivalent fully priced package.
There's nothing unique about the Essentials package advertising that is designed to put people off "because it is costing them too much".