The target market for this product will either not care what platform the screenshots of the cross platform tool are taken on, or they'll get a negative impression if it is not OS X. Therefore, OS X.

The software is cross-platform. I've only used it on Windows (I have emacs on OSX and Linux) but I have Mac friends who rave about how smooth it is there too.

It might be strange, but many of the engineers on the VS Code team do their development work on a Mac or Linux PC.

Not only that, but the demo video they show on their webpage is recorded on OSX.