Hi Jim and Joseph,

I believe that the sentence should read: "On ne pense qu'à soi."  

Or: "On ne pense qu'à soi-même"  

Both Larousse and Collins dictionaries list "soi-même" as a personal pronoun. 

Here is the entry from Collins Dictionary:

Pour l’apprendre aux autres il faut le savoir soi-même. To teach it to other people you need to know it yourself. ⧫ To teach it to other people one needs to know it oneself.

soi-même [swamɛm ]

PRONOUN oneself

Il vaut mieux le faire soi-même. It’s better to do it yourself.

Hi Joseph.

Soi-même is not a stress pronoun.

Bonne continuation.

Jim 

Bonjour Joseph,

"Soi-meme" is indeed a stress pronoun. 

Here is the link to the French Grammar Topic on Stress Pronouns in Lawless French:

And, here is the link to the Lesson on Stress Pronouns using meme:

French lesson: Using [French stress pronouns] + même(s) to express "myself/yourself/etc" in French (lawlessfrench.com)

I suspect that your sentence was not accepted because it should read:

"On ne pense qu'a soi-meme." (vs. "On ne pense que soi-meme.") When using "penser" to express to "think about/of someone" it is followed by "a". 

When expressing an opinion of something/someone, "penser" uses "de". 

As in: "Que penses-tu de ce roman?" 

I hope this helps.

Bonne Continuation !

P.S. You may have to copy and paste the links I gave you.