"The decision to keep Hollow closed source isn’t about restricting users, it’s about ensuring stability while maximizing customization through plugins and themes."

I am not sure if the above is marketing speech but of course you can open source it while not accepting PR that would harm the stability of the app.

Most developers interested in such software would not invest time and effort towards projects that may be abandoned soon, or that may have their future versions require subscriptions.

I am not going to be using Hollow, it is unusable in its current state and I do not see any security in helping the app grow.