The potential for a video to go viral cannot be accurately measured by a single metric alone, which is why it is complex.
In the scenario mentioned earlier, the video in question was a stitched video that already managed to capture the attention of users. If my video is 5 seconds long and has a 60% retention rate, resulting in 300 hours of watchtime, your 1-second video would have a 95% retention rate but only generate 40 hours of watchtime.
If you were TikTok, which video would you choose to promote on the "For You" page? Would you go for the one that keeps the audience on the platform for a longer time or the other one?
That's why I mentioned that the viral potential is the same if you know what you're doing. If your 8-second video ends with 3 seconds of nothingness, your retention rate will decrease, making my video more significant based on that single factor (because it's the same viral video stitched together). If all the other metrics are similar, but I have an 80% retention rate while you have 50%, it's evident which one is more likely to succeed.
However, the case you mentioned is not exactly the same. We won't have similar metrics, so TikTok will obviously not prioritize something that generates only 40 hours of watchtime after 400,000 views.