Post gone viral, 16000 visitors in a day, how many actually read the article?

Edit: A few people have pointed out that my assumption about the Analytics engagement metric might be wrong because single page hit could be counted as zero on engagement time. I'll make an update to this post when smarter people than me on HN can agree on a metric. So I open this problem to Analytics expert, how can I discern readership ratio from Analytics data?

My reminiscing post about my time as an aerospace engineer versus software was on the front page of Hacker News for about 12 hours on Friday. That garnered 16,374 unique visitors to this site on that single day. However, Google Analytics data say that only 975 of those people spent more than 10 seconds here. Given that there's 652 words in that viral post, I doubt anyone can actually read it within that time. If we assume that only people spending more than 10 seconds have meaningfully read the article, it appears that only 6% of traffic are real readers from this Hacker News blitz.

Viral post visitors engagement

Given that my usual stats is above 10%, viral traffic audience is understandably less targeted but isn't abysmal by comparison. However, as a data scientist, I'm obliged to say that as this is an one-off event, we couldn't draw a statistically significant observation from it.

Interestingly, overall traffic the day after on Saturday is back down to 901 visits. And engagement for those spending more than 10 seconds is up at 8.3%. These residual traffic are coming in from domains like Twitter and link sharers.