Major exaggeration. If lighter fluid produced a high that was anywhere near as satisfying, no one would take the tremendous legal and health risks associated with shooting smack. A few might turn to lighter fluid, but I'll bet a huge number of them would just be sober.
You don't know junkies if you think that they stop seeking a high if they can't get the more traditional drugs. It is not an exaggeration at all. There's a reason legal highs liek Spice and other cannabis substitutes have such demand, despite them being much more dangerous than cannabis. People will seek out what they can get.

Lighter fuel will indeed get you high. It might not be as satisfying as heroin, but if it's all you have, and you're a junkie, it's a matter of time before you're addicted to lighter fuel.

Of course there will be less addicts. If you remove the substances that are most addictive....there will be less addiction!
There will be less heroin addicts, but the number of junkies will remain the same.