I don't think I would trust anything I read from anyone today. That being said, I doubt casinos will be making fake offers and then claiming April Fools. People are pretty particular when it comes to their money and in an industry like gambling I could only imagine the uproar from users if a fake offer was presented as a joke. While it's best to take anything you read today with a grain of salt, I especially wouldn't be making financial decisions based on anything you see. My advice is to just go about your business as normal and refrain from getting excited about any special offers that appear too good to be true. Some people have pretty bad taste when it comes to things like this.
That will always be the doubt, and it do makes sense. However, this day (April Fools Day) is still celebrated on some other ways, some casinos are giving legitimate promotions to deposit/play, etc. Memes competition about it are also done and they really are giving away rewards.
I guess we can still assess if an announced "promotion" is just a joke or not.
But yeah on top of it, some casinos are probably abusing this day by giving fake promotions, and if the user won, he won't have the prize since it is just a fake promotion.