I don't think that's how Obi-Wan deduced to leave. I remember that line of the book quite clearly:

It would be mercy to kill him.

He (Obi-Wan) was not feeling merciful.)

Or something like that. Now I have to go check. haha.

Ok, yup. Obi-Wan senses the shuttle approaching...:

Yoda had failed. He might have died.

He might have left Obi-Wan alone: the last Jedi.

Below his feet, Darth Vader burst into flame.

"I hate you," he screamed.

Obi-Wan looked down. It would be mercy to kill him.

He was not feeling merciful.

He was feeling calm, and clear, and he knew that to climb down to that black beach might cost him more than than he had.

Another Sith Lord approached.

In the end, there was only one choice. It was a choice he had made many years before, when he had passed his trails of Jedi Knighthood, and sworn himself to the Jedi forever. In the end, he was still Obi-Wan Kenobi, and he was still a Jedi, and he would not murder a helpless man.

He would leave it to the will of the Force.

He turned and walked away.

After a moment, he began to run.

He began to run because he realized, if he was fast enough, there was one thing he could still do for Anakin. He could still do honor to the memory of the man he had loved, and to the vanished Order they both had served.

So it's a whole bunch of things, really.