In August 2015 they told us what content was coming in December 2015. Following your logic they would've told us in 2012.In September 2015, Frontier was indicating the content that was coming in May 2016, October 2016 and April 2017. My point is obvious, Frontier know what is coming in late 2020, they must have done the bulk of the work, they choose not to tell us, probably for marketing reasons.
Well obviously they know which content is planned for late 2020, they are working on it since two years after all. There are several reasons they don't tell us yet, marketing, expectation management and potential changes to the planned content.
For some reason you claimed that the current situation is somehow different to the marketing period before the Horizons launch, which is not the case as I've shown several times. That's completely unrelated to the point you want to make though, you don't need to compare Big 2020 to Horizons just to find out that they know what they are working on. Assuming anything else would just be completely idiotic.