Was Splinter of the Mind's Eye proof that George Lucas was making things up as he went along?

But it was written by Alan Dean Foster, not George Lucas, so it doesn't tell us anything about Lucas's plans for future films.

And we all know that Lucas was making it up as he went along.

Splinter was never considered canon in the films.

According to Wookiepedia

Splinter of the Mind's Eye was written by author Alan Dean Foster (...) It was based on story discussions with George Lucas, and it was written while Star Wars: Episode IV A New Hope was still in production.

There is two ways to interpret this situation.

One is that Lucas had the whole trilogy planned in his head, and threw a bone to Foster by feeding him already scrapped ideas.

Two is that Lucas narrowed it down to two or three possible plots for the sequel. After they had their discussions with Foster, he kept on developing a different scenario for the next movie.

I think both of these are understandable. During production of EP IV they did not know it will be such a runaway hit, so the sequel plots were not finalized, which I think is quite normal.

From what I've seen, Splinter was Lucas's back-up plan. He'd already secured the rights to the sequel in his contact for Star Wars. He gave up points and upfront money in exchange for rights to merchandise and copyright on all material and sequels.

However, Lucas wasn't sure Star Wars was even going to be a success. So with Foster he developed a script for a very low budget sequel.

As for making things up, I'd assume when it comes to creation and writing most people have a general outline and grow their story around that. I don't think it was any different with Lucas. If you look at early concepts and ideas for the original movie it was massively different than what he ended up with.