It's not my website, but my question is: how could it generate millions of traffic in such a short period with a low number of backlinks and such a high number of articles published? And how could 19,000 pages be indexed in such a short time?
it's probably a domain that google trusts (good expired / non-dropped domains are very sought after for this very reason)
Anyway, take a look at which keywords and pages get the most traffic, maybe you can figure something out that way, because it's possible they're seasonal keywords that no one targets and - with a well optimized page on a quality domain, and with no one else targeting those keywords - it's possible that google rewards this site / page specifically, which would justify why it gets so much traffic in a short period of time.
So, look into this site's top performing keywords and pages more closely. Also look at the competition for those keywords too, because - like I said - if the keywords are seasonal and have high traffic volume but the competition is low / have poor content then this could explain the high traffic.
But if the competition is tough and this site still got so much traffic but the page(s) that get all of this traffic "don't have" backlinks, for example, you can draw the conclusion that these pages have hidden (but strong, of course) backlinks pointing to them, otherwise they wouldn't be able to pull so much organic traffic normally...
From that screenshot we can only speculate, so you have to dive deep into analyzing everything (keywords, authority, pages, competition, etc) if you wish to find out the secret