Aseprite is open source. The source is open for anyone to access right here: https://github.com/aseprite/aseprite

You might be confusing license with access. The product itself has a proprietary license. Even then, a majority of the libraries they produce are also available under the MIT license.

"open source" has a specific definition[0], which this project does not meet. When people say "open source", that is the definition that they are referencing. It's the reason why there's been endless discussion about "open weights" models not being "open source".

"source available"[1] is a different thing, and you're right that this project is "source available".

[0]: https://opensource.org/osd

[1]: https://en.wikipedia.org/wiki/Source-available_software

Source available is not open source. Don’t try to redefine what open source means. It’s so insulting to volunteers hard work.

How can you say its open source and 3 sentences later that it has a proprietary license.

Their EULA forbids distributing the software, hence not open source.

You are describing source available. That is not the same as open source.