What are you talking about? The parent comment literally linked the GitHub repo with the Aseprite source.
> (c) Prohibition on Reverse Engineering, Decompilation, and Disassembly. You may not reverse engineer, decompile, or disassemble the SOFTWARE PRODUCT, except and only to the extent that such activity is expressly permitted by applicable law notwithstanding this limitation.
And more. Doesn't seem FOSS, more like "readable source" or similar.