There is a limit for HDMI and DP according to versions. Also for VGA and DVI. Your graphics card supports VGA with a DVI port. And it looks like there's no limit, but limits exist. The limit I wrote is for graphics cards with VGA output.
You're talking about the 400mHz limit (2048x1536@85hz is technically 396mHz), which is stated in the specs. But that can be easily patched then the analog DAC will go beyond 600mHz, with no signal degredation I could detect.
Either way, the 400mHz limit is still leagues beyond what most external adapters can process.
I don't have the experience you wrote. However, the example is only possible with DVI output. And RX5700 series does not support DVI output. Your monitor probably comes with DVI support and gives you the experience. I haven't tried OC using a VGA port. Converters cause loss of quality. You may feel it or not acording to coverter quality.