It's true that I get this message for downloading folders with nested folders inside, but downloading folders with only files inside goes through but ends in a corrupted file. This is also true for empty folders, which are technically one file.
As a side note, and as you may have guessed, yes, I am trying to download my world off of Minehut after not using the service for many years. I'm not impressed that this single-file limitation exists and it has thoroughly convinced me not to spend a single dollar for any reason. For the sake of users who simply want to download their worlds for archival or security purposes, I would appreciate some compromise be made here.