A few employers ago, giving a "best guess" rather than admitting you don't know the answer to a question was an instant no-hire. They wanted to be sure that you were sure when you put an idea out there without qualification, and so it was with the day-to-day SRE-ish work where short outages caused by speculative changes that weren't fully understood were wildly expensive events.
At my most recent one, I was actually encouraged to speculate if I didn't know the answer to a tech question, or was asked why I answered what I did if the answer was wrong; they were more interested in my thought process than the end result, even if the end result wasn't technically correct.