I mean, it seems a pretty open-and-shut case. The author literally admits he copied the code. From one of the comments he makes:

> @kuckir, to be clear about the lineage: PhotoSuite started from the Photopea build mirrored at https://github.com/ruanjiyang/Photopea-Offline, and the README credits Photopea as the primary inspiration and prior art. Over the past year it's been rewritten and rearchitected module by module, retargeted for desktop, restructured into ~330 source files, with a lot of features Photopea doesn't have. It's a rewrite of that lineage.

That Photopea-Offline is an unofficial rip of the actual app. Pretty sure using a pirated version doesn't suddenly grant you license rights to use & extend the code into another competing product...