I've wondered how it's applied. Once there was a super old thread that had like 2 new posts within a couple minutes from bots, and it was Super Hot. I have 2 theories:
1) Some algorithm takes into account how many posts have happened on a thread, and the older the thread, the fewer posts it needs to get Super Hot. Hence a thread a few days old consistently getting a couple posts/hour could get it.
2) It looks at how many people are opening a thread. Some threads get Super Hot without having a ton of posts, and it could just be because lots of people are opening the thread without posting in it. There were a ton of upvotes on the first page which means a lot of people were reading it, even if there weren't a ton of posts.
It could also be a mix of those, but idk if it would be that complicated.