I understand being suspicious, I've played the top 10 games, and most people would be blown away if they found out that they were made in 48 hours.
But GDevelop jams allow users to pilfer work from their previous projects, example games, and even use purchased or publicly available assets on their games, all to try and create the coolest thing they can within the allotted time period.
With that much freedom, and the instruction to "start from a starter", which come with the controls and basic mechanics for the genre of game you want to make, I'm not surprised by the quality of the games that people made.
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We do disqualify games if they pilfer too much work from one of their previously existing projects/a game example, or if we find out that they were creating assets/working on a project for this jam before the start time, but the games needing to align with the jam's theme usually prevents people from doing that. (Since they could spend a bunch of time working on something that completely doesn't align with the theme, and would have to toss it all out.)