Start with the day table, not the heatmap. Day of week is the bucket with the most videos behind it, so it is the number least likely to be noise, and it is also the one you can act on: a channel can move its release day far more easily than it can move a specific hour.
Then read the hour table as a window rather than an instant. The tool buckets by the hour a video went live, so "9 AM" means the whole 09:00 to 09:59 window. An upload fifteen minutes either side of a good hour is the same upload.
Treat a single strong cell in the heatmap with suspicion unless it appears in the recommended slots list, because that list is the only part of the page that enforces a minimum sample. A dark cell holding one video is one video.
Finally, weigh consistency above the optimum. The subscriptions tab and the notification bell both reward a schedule an audience can learn, and a channel that holds a merely good slot every week beats one that chases the best slot and misses.