I think in this case the problem is that Inkscape's rendering pipeline is just extremely computational inefficient on macOS. It's not that things don't show properly, it just takes too long to draw all the pixels. MS Excel for Mac actually has the same problem if you fullscreen a spreadsheet on a 5k external display unless you put the display in low res scaling mode.

If I remember correctly, which I probably don't, it's partly also because the macOS display process involves painting something like 2-4x as many pixels as the screen actually has and then downscaling the result because that looks prettier than just drawing the exact pixels directly. But I'm probably misremembering that part.