That does not mean that the GC did not write a letter. In most cases, GCs do write letters for students as part of the SSR.
Yes, they can check the box stating that they are not providing a written evaluation.
They can also request a telephone conversation to talk about the student.
As a person who has written many letters, I checked the box stating that I was not writing a letter was when I knew that writing a recommendation would be challenging or I would be submitting it with reservation.