Well, it’s given in question that robot can start at any time and not simultaneously i.e., according to the above-given case if robot at cell 4 start at ‘t’, it isn’t necessary for the robot at cell 8 to start at ‘t’ it could start at ‘t+4’ also.
“But the catch is that each of the robots can start their journey at any time. They don’t all have to start at the same second. It won’t stop once it starts moving though. And they all move at the same speed of one cell per second, once they start.” - quoted from question
In this question it doesn’t matter if there is a way for robots to not collide, what matters is if there is a way for robots to collide. If there is a way for robots to collide then it is unsafe, if there is no way for robots to collide whatever time they start then it is safe.
I hope that cleared your query.