It makes sense they would make this a free PDF, since Frostgrave II is due out in July of 2020. Sort of like Warlord did for the Black Powder rules.
Also, it seems that Osprey's site has been a bit on the slow side today, maybe due additional traffic that might be associated with this offer. I went on the site and placed an order for the PDFs listed, but a quick heads up ... Frostgrave and Dark Alchemy are both free with the code FGV2020, although the shortened version of Frostgrave: Perilous Dark is not free from Osprey, and instead available from the link/download provided on the blogspot post that 023 gave us above.
I'm buying the Perilous Dark pdf from Osprey anyhow because I am curious about the rest of the scenarios provided inside it.