Many open source licenses levy restrictions upon the acceptable use of the software. Those restrictions may include attribution requirements, up to and including a requirement to include the license when redistributing the code; they may forbid using derivative works for commercial purposes; they may require the downstream project to utilize the same license. Open source is not the same thing as "anybody can do anything they want forever."

Papermark is AGPL; Corgi must release all its changes.

It's really hard to not assume this is intentional ragebait.

A cursory look reveals they aren't complying. So, as you say, they are stealing. What's the point of this comment?

Copyleft is still a thing. Right to attribution is still a thing. Please, read about it and you will discover that there is a lot of nuance to the open-source code.