That is a bit of an oversimplification.
Hello Games is using Steamworks for NMS’ multiplayer component like several other games do, so they were able to release the update to Steam users at launch. GOG can’t access this framework, meaning they have to redo the multiplayer parts that are platform-specific in order for it to run on GOG Galaxy, which they will be using as the back-end.
Steam’s API is very easy to use in the sense that it does a significant amount of work for you, whereas it can be assumed GOG doesn’t have this kind of tool, hence they’ll have to come up with another solution. It makes sense why Hello Games would choose Steamworks for the multiplayer component with its ease of use, on top of Steam being the most popular platform out of the two.
This post from a month ago goes into a good discussion about Steamworks vs GOG in the comments section.
You are right though, GOG are a class act and considering they are run by CDPR, that explains it. On the other hand, the refund is only available in the form of in-store funds. Several GOG players want to take their money back so they can use it to buy the Steam version instead, however they can’t this way. In an ideal scenario, Hello Games should give GOG players who own the game Steam keys, so they can play it there.
Delaying the update entirely so GOG users can get multiplayer at the same time would’ve pissed a bunch of people on other platforms off. Offering extended refunds for a delayed update, while a nice gesture, the game has been out for almost 2 years and the refund is locked to in-store credit, so they can’t get another version of the game that has the multiplayer component at launch with that money.
At the very least, Hello Games should’ve warned GOG users beforehand that they wouldn’t be able to get multiplayer until later in the year.