I'm not really following. His rationale is that Congress picking one of three terrible candidates is better than the voters' doing so?
As I understand it, he thinks Johnson would be meh as president, but that the nation can recover from Congress selecting one term of meh easier than voters setting themselves up for possibly two terms of terrible. I don't think he's left-wing (he's just out of the military and more vocal about Trump than Hillary), but I thought about him because of this line: Replace "left-wing" with "non-damaging". He sees a narrow path in November where voting third party could increase the chances of a non-damaging outcome. He doesn't think that result would be illegitimate because the pathway to the outcome has been around for a long time, and no one has cared enough to plug it.Voting for a third-party presidential candidate must therefore either (1) be purely symbolic or (2) increase the likelihood of achieving left-wing outcomes even while losing.