Ideas can be frightening, and can hurt those who produce them.
See for an example the ideas of Galileo, and where they got him - house imprisonment, banned from discussing the heliocentric view. Science fiction, on the other hand, can be frightening, but it is contained. It is fiction. Yet still, like all literature, it contains an idea - it conveys the author's point, but more subtly than outright stating it.
Samuel R. Delany [says],
“The variety of worlds science fiction accustoms us to, through imagination, is training for thinking about the actual changes—sometimes catastrophic, often confusing—that the real world funnels at us year after year. It helps us avoid feeling quite so gob-smacked.”
It conveys the idea, more subtly than explicitly stating it.
If the author is asked about that idea, they can say that it was merely to explore, merely to write a story for people to enjoy, by no means to propound an idea. By nature, it is a way for a writer to explore an idea, but with less impact on themselves. Le Guin also said in the quote you provide
Writers may find the future appealing precisely because it can’t be known, a black box where “anything at all can be said to happen without fear of contradiction from a native,”
Precisely because it can't be known, where anything can be said to happen without fear of contradiction. Because the future can't be known, we can talk about in the format of a science-fiction book without fear of repercussions.
Also interesting is a later quote in the article:
“I really like design fiction or prototyping fiction,” says novelist Cory Doctorow [...] "It’s like an architect creating a virtual fly-through of a building.”
It's also safe in the sense that nothing is going to blow up while writing - it's all on paper. It's an exploration of what could be that examines the consequences, before it's built.
So much for the term safe; now for laboratory. Laboratories are places where experiments are done, tests are made, theories are explored. Likewise, a science-fiction novel is a laboratory for the future. What if this is true? How does that affect the plot? Look at some of the questions on Worldbuilding.SE and you will see that the world that is constructed imposes restrictions on things that can be done in that world, and it affects how the people in that world operate.
And finally, the term that is I think the strangest in that quote, sterile. The fourth definition of sterile as per Webster is
"fig.: Barren of ideas; destitute of sentiment; as, a sterile production or author"
So in this case: a laboratory barren of previous ideas, so it is therefore open to the creation of new ones. If you want to explore the absence of government, you can write about it while ignoring previous stances on the subject. You can introduce out there things like warp drives. It allows you to be more open to new ideas by taking them away from the backdrop of the old ones. As the book The Storytelling Animal: How Stories Make Us Human (google books link) says:
fiction seems to be more effective at changing beliefs than nonfiction
Semi-recent examples can be found in the Mary Tyler Moore show or the Bill Cosby show, which improved how people thought about working, single women and African-Americans respectively, in a fictional format. With this in mind, the science fiction novel is a laboratory that is sterile, totally clean, from other ideas and impressions. The world can be born anew in it.
So, my overall interpretation would be: a science fiction novel is a place to explore ideas about the future in a way with less repercussions and with less interference from current ideas than a non-fiction book or opinion piece might have.
With regards to the critique of the original article, I'd point out this quote, which sums it up:
Sci-fi, then, doesn’t just demonstrate future possibilities, but future limits—the extent to which dreams of what we'll do remain captive to the things we've already done.
With this in mind, I see it less as a critique of the original article/quote and more as an extension of it. Sci-fi is still a safe, sterile laboratory, but it's being used to explore limits as well as possibilities.