I'm sure one can design a VGA switchbox in many different ways, internal or external.
I choose to use an off-the-shelf switcher IC and a single wire that is either high or low to select source (and build in a UBE switch for the Atari source while at it, because why not?). I'm not sure how you would design it any simpler than that but in the end, equipped with a soldering iron and some low-level code you can do pretty much whatever you feel like doing
This software doesn't really have much to do with any specific hardware though. It's not attempting to be a switchbox, but it does toggle a hardware pin inside the Atari that you could potentially hook up to one like I've done.
What the program does is allowing TOS to run ST-Low resolution color games even if the machine is currently running in Mono mode (if you have a UBE-switch), or even if the desktop is being displayed using special hardware such as the ET4000 or the LaceScan.