Source available proprietary software is readily inspectable, but often not legal to modify (at least not for free). It might still be extensible (ex plugins) though!
Closed source proprietary software is difficult to inspect, difficult to modify, and generally illegal to modify (for most use cases, in most jurisdictions). As above, it could easily be extensible though.
On the other hand, a well documented plugin or scripting system, which sits on top of the existing domain logic and is well documented and full of examples, generally is an excellent way to allow extending the base app.
The base app can be open or closed source. Without scripting or plugin system it's still a black box for most intents (as the time needed to study and change it would likely be too much).
As someone who writes plugins, being able to see "behind the curtain" is very helpful, especially for taking advantage of un-documented features.
please don't say the wording is the problem. "as extensible as possible" is fine. "as possible" is a qualifier, it clearly means "as extensible as possible without undermining other goals". you're pretending that they've promised to make it extensible at the expense of _everything_ else, and that assumption in context in unfounded.
it's true that paths to monetization exist for open source software, but they usually aren't accessible to an individual developer who is building a small bootstrapped side-project to generate small amounts of passive income.
setting that aside, though, the best way to make software (open- or closed-source) extensible is via a plugin API. without it, the only way for end users to extend open-source software is to fork and maintain the fork forever, or attempt to merge upstream. so one might argue that the quality of the plugin API (and its documentation) is the primary measure of a software's "extensibility".
The app would need to be open source for this to be possible.
It's still possible to make money with an open-source product. When you're targeting a developer audience, it might even be more profitable to be open-source.