Since around the end of 2018, sections are no longer special tasks, but they are real separate entities, you can read that post about it.
Sections are dead! Long live sections! - Developers & API - Asana Community Forum
Now, if you need to know which section a subtask is in, you can read its “memberships.section” opt_field.
But, there are good chances that it will be empty, because most of the time, the “parent” task is in a section, but the subtask is not.
So, you can query opt_field “parent.memberships.section” of your subtask, to get its parent section.