An actual explanation as to what happened from what I can gather
Humble uploads image files containing the lineup to a email host server for when they distribute the emails to newsletter subscribers. Since they follow a traditional format, people just changed the image source to the next month a bit early and then used it to figure out the titles for the source.
Last month (and evidently this month as well), leakers started to directly post the image file as proof of the leak for clout (usually the leaker that posts the leak first gets a lot of reddit karma/twitter likes). This probably caught the attention of Humble, who then decided to upload a fake version of the April titles under the usual format as a way to bait out and confirm the leak source, while uploading the image of the real titles with random character strings attached to the file name, making it impossible to guess. Up until this point, Humble never changed the format of image uploads like this, so many of the reliable sources verified that this leak was real because Humble never changed their patterns up until this point. This wasn't an april fools joke but rather Humble managing to successfully identify and kill the main source of humble choice leaks. Very hilarious, especially since the only reason people got to this point in the first place was because people posted the images for clout instead of simply just naming the titles.