The claim about open-source is coming from the shareholder of Organic Maps, Alex, but there is no basis to it, as the code for that repository is licensed under the Apache 2.0 license.

It appears Alex is angry about the fork and doing anything possible to spread negativity.

The code used by the fork was never published. It was stolen from a private repository and a private server, and then published/used in the fork without the authors' approval. That's a serious legal issue.

The fork also took the new website design that was developed for Organic Maps even before the Organic Maps website was updated.

Don't believe everything on the internet; there are many lies spread around.