He’s saying the script with dependencies relies on steam. GOG’s runs offline. But like you said, copying the end result is generally fine (and especially so on Linux where it’s all contained in the fake folder structure).
If there’s not some moderately heavy DRM on Steam, you’re more likely to get the same build on both stores (not always, though). It’s when GOG is actually the only DRM free version that you tend to end up with a lack of version parity.
I talked about the
substantial enough to need an installer
line. Like what makes a game substantial enough to need an installer ? Steam and every other game launcher with install capabilities is more or less just a fancy installer. There is no more effort needed for a publisher to generate a new installer binary than it is to generate a new steam patch. Even if gog installers are offline it’s more or less an archive with a binary stub to unpack it and the install script. This one is on the publisher and not on gog. And for the version difference, do you have an example where the gog and steam versions differ ?