Did somebody in the room say, "We've crunched the numbers, and our best estimation is that if we orient all our resources toward achieving this goal and put forth our best possible effort, we'll be able reach 5x."
And then did somebody else in the room say, "I agree. So let's be extremely audacious and shoot for 10x"?
Either the first person was miscalculating or the second person was being unreasonable.
If you know how to do 4x, but think 10x is epsilon away from “best possible”, you set a 10x objective even if you only know how to hit 4x.
As per the spirit of parent's question - either you're assuming something impossible, or you're setting a goal to achieve something that is possible (in which case why haven't you scoped it out clearly & sanely).
Purposefully choosing goals that you don’t yet know how to achieve is one of the powers of OKRs; they’re not meant to be limited to project plan-style marches towards a known outcome.
Take the classic YouTube “one billion hours in daily user watch time” by the end of 2016, which was about a 10x multiple over their 2012 stat. There’s no way they knew how to reach that goal, but the objective helped them to align and constantly make small decisions in support of that goal (instead of other conflicting goals, each of which could be argued to be good for Google in a different way).