Several threads in the forum cover this. Maybe take a look and tell us what you find?
[Week 3 Yolo Doubt About Sliding Window - #3 by ai_curious]
[Quick question regarding YOLO algorithm]
[Detecting Multiple Objects using YOLO - Grid Cells plus Anchor Boxes]
The tldr is that grid cells in YOLO are not sliding windows and unlike sliding windows, YOLO does not actually divide up the input image into subregions. The grid cells represent sets of predictions, each of which is made concurrently and each of which uses the entire input image.