Having watched our beginner GDScript tutorial or read the GDScript intro in the official docs is a pre-requisite for this course, and the feature is covered in both guides.
Also, is it an issue for you? You seem to know inferred typing already. I add extra information or lessons to solve problems that multiple students actually struggle with. Otherwise, I prefer to answer individual questions individually.
I skimmed the GDScript intro and missed that part, and it took me a little to figure out what := meant. I was thinking about Pascal assignments at first but then was wondering why it wasn't used everywhere. Anyway, fair enough as you do state that the GDScript tutorial is a pre-requisite. Cheers.