So Apple added Javascript OSA support without me even noticing. Guess I've been using the same scripts that just work for a very long time.
One thing I like about Affinity is that they don't just go for the lowest common denominator for the platforms they support. They produce first class mac applications that use the appropriate technology on the platform – look at their home page which show cases all the Apple technologies they use: Grand Central Dispatch, Core Graphics, Force Touch, Retina UI, iCloud Drive, Metal 2, DCI-P3 panel support.
When they ported Designer to Windows, it wasn't just a normal port, they made sure it used current Windows development best practice too. This gives me hope that they will indeed support OSA on macOS.