Here's your proof that the tweet is false:The existence of GoTY edition proves that the tweet was correct. Keep trying.
It's also important to note that GOG Galaxy is able to distinguish between the standard and Game of the Year editions of the game. You'll get the equivalent one as a freebie. That means, if you've got the standard edition and have bought the expansion pass separately, you won't get the GOTY edition from this promo. You'll have to buy Hearts of Stone and Blood and Wine again.
Own The Witcher 3 on PS4 or Xbox One? You can claim a free PC copy from GOG
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www.eurogamer.net
Eurogamer is a UK site. They clearly mention that owning the standard edition edition + expansion pass separately does not entitle you to the GoTY edition. What further proof do you need?