The miniseries was released over a little more than a month — five approximately one-hour episodes. The creators also have a podcast for each of the episodes where Mazin explains minor deviations from historical truth and artistic decisions. For instance, he deliberately placed Legasov’s suicide in the opening scene.
Chernobyl's new sarcophagus took two decades to make. Bigger than Wembley Stadium and taller than the Statue of Liberty, it will seal in the entire disaster site for 100 years. Chernobyl's renewal
I had the exact same question when I saw the episode. Building on user1476176's answer, I have thoughts as to how someone might have derived a much higher upper bound, though until someone pulls the actual source they used we won't know. They did explicitly say in the episode that other reactors would be engulfed, presumably with their water.
I4IZ3.