They’re looking for something open-source. Draw.io’s readme says:
License
The source code authored by us in this repo is licensed under a modified Apache v2 license. This project is not an open source project as a result.
I haven’t been through the license to see what its restrictions are, but there must be a reason they give this warning.
$20 per month would be enough to discourage me. It’s another relatively costly computer-related subscription and I already feel like I’m losing a battle to keep those minimal. There would have to be some very clear benefits for that price.