It reads the most recent page of posts, not your whole history. For an account posting daily that is roughly the last month; for one posting weekly it reaches back much further, and older posts were competing for a different audience than today’s. The date range the analysis actually covered is printed with the result, and it is the range the answer applies to.
It measures when you posted and how those posts did. It cannot separate the hour from everything else that differed between them: a Tuesday evening slot may look strong because Tuesday evening is when you happen to post your best-prepared videos. The heatmap is evidence about your account, not a controlled experiment, and the honest way to use it is to test the top slot against your current one for a fortnight.
The timezone line is a guess, and it is labelled as one. It reads the five quietest hours of your posting week as the middle of your night. When your posting is spread too evenly for that window to stand out, the tool returns no timezone at all rather than a number that would look like a reading.