In Canvas, users (with the exception of lecture section instructors) are only allowed to see people enrolled in the same section.
Limiting students to "interact by section" only affects Collaborations, Chat, People, and Conversations. These limitations allow TAs and instructors to grade students in their same section(s).
Discussion topics and Pages are not affected by section limitations and can be viewed by any student. These feature areas could be restricted by creating content in course groups.