That's not really the argument he/she's making though. If the vote is won by only some small percentage, then everyone that voted in that election's vote mattered since by making the choice to not vote could have caused the other party to win. What that percentage is depends on a variety of factors, but is somewhat irrelevant to the point.

Their point is that if it were the case that California was a close race and that the electoral college is close such that Trump winning California allows Trump to win the election then it is important to vote for POTUS in California. However, those things aren't statistically independent, and it is incredibly unlikely to happen.

Yeah, the biggest thing the whole "Your vote counts" thing aims to fight is widespread voter apathy.

I don't think it's effective. Anybody with any understanding of statistics can easily see that your vote doesn't affect the outcome. I think it might be more effective to frame voting as some sort of moral obligation -- "people died so you could vote" "you live in a democracy" sort of rhetoric.