The problem here is that within 24 hours of release, Hello Games was putting out all kinds of statements about how they were going to fix all of the issues people were having and how the "servers" were "down" due to the massive amount of simultaneous players the game had on release. It's not unreasonable to assume on that basis that there WAS a multiplayer mode and that it merely wasn't working because of server capacity issues.
A reasonable player seeing the statements that Hello Games made could easily have been fooled into thinking "Oh, there IS a multiplayer functionality in the game, but it's down and needs a fix before it works", played past the Steam refund limit (2 weeks/2 hours played) only to find out after they'd already gone past the threshold that there was no multiplayer and never was.
It would be one thing if the developers had said, unambiguously, "There is no multiplayer or online functionality at all in No Man's Sky, please do not buy the game if this is what you're looking for", but when they were making statements like "The servers are down because so many of you are logged in at once!" post-release, it creates a grey area.