A small percentage of people would use the word 'computer' to describe a smartphone, let along a featurephone.
Imagine you ask a random person one of these questions:
A) Do you own a computer that has a touch-screen?
B) The computer you use most - how big is it?
C) Do you have a computer that can make a phone call to a landline number?
If the most common answers are:
A) Yes
B) In the range 4" - 7"
C) Yes
Then I would agree that the definition of 'computer' includes smartphones. If it includes smartphones, it might also include featurephones.