So in this case they cited "no client-side routing wiz-bang" -- that's one of their opinions and won't be introducing any support for that sort of thing.
The classic example is Python's motto "There's Only One Way To Do It", which was in a response to Perl's "There's more than one way to do it."
Another example is Ruby On Rails...it has a strong opinion on how the application should be structured -- must be MVC, most of your code should be in a model, convention over configuration, etc.
It is "opinionated" in the sense that it has an "opinion" on what's the right way to accomplish a certain outcome, and thus it preselects/premakes some choices for you.
The implicit tradeoff is that it may not be as good at general problems / all approaches, but may be better or easier or both for specific goals / approaches.
I never took "opinionated" software as "no configuration". For me it's always been "custom defaults". It might provide some configuration, it might not, but what makes it opinionated is the fact that the defaults are not generally applicable, but rather tailored for a specific use case.
That said, I think it'd be really difficult to work around Elder.js' opinions.
It allowed for highly complex scenes, great static lighting and short loading times but it lacked in detail and dynamic elements.