The answer is pretty simple yet vague, they know folks on the inside in some capacity. Sometimes they have to do a little figuring out from what info is given to them.
Either way, the speculation maps from HHNightmares are the most reliable of the spec maps out there. They do update the maps regularly when new info or the info they have becomes more clear. House-wise, everything but the Shrek house is pretty much locked in. Especially for BoF, Tooth Fairy and Beetlejuice as all three have opened in some capacity last year. Can also say the same for Puppet Theater since that was leaked by Uni/Peacock a while back.
I'd be expecting another update to the map eventually, especially since there's a chance the Shrek house might actually be Halloween Kills instead. Something that's no real fault of the insiders since it was generally accepted that the new trilogy was off-limits...so much to the surprise of everyone when the trailer for HK showed the masks from Halloween III.