As a mathematician who has studied voting systems, I can tell you that that statement is completely wrong. A vote for a third party is mathematically just a vote for a third party — in no possible way an indirect vote for R and D.
If you don’t believe me, try it yourself with a little model with a few voters.
Also, most Americans do not live in swing states. Mathematically, their vote for R or D is wasted in those states. If you don’t live in a swing state, you get the maximum power for your vote voting for the third party that most represents your interests.
But we certainly agree on one thing — voting for Trump is a terrible idea. Clinton is a wretched candidate, but not nearly as terrible as Trump.