I mean, there's still some Rebellion-era canon that could use forgetting (coughSplinter of the Mind's Eyecough) but it's not nearly as rampant as it is in the later EU.
It is. I mean, don't get me wrong, I enjoy the book! It's fun! It just has no place in Star Wars canon because it doesn't fit anything that came after ANH.
I'd guess he wasn't given any appropriate guidelines - kinda like the Star Trek novel that Vonda N. McIntyre [!!?!] wrote that time. Or Spock, Messiah.