This is absolutely a myth.

Green party candidates can run and win many tickets other than President, but it's very, very hard to get ballot access, public funding, or voter awareness. In any state that isn't a swing state, voting Green most likely has more actual consequence than voting for one of the two major parties. This builds infrastructure that lets Greens organize the way parties do in other countries: doing actual outreach between elections instead of just threatening people every time they need votes.

The Green party is also often the only way to actually challenge the duopoly when both parties are taking the same pro-corporate position. What they do in a race is break a cartel dynamic, which forces Democrats to actually adopt popular positions which they can then be pressured to act on.

Vote strategically: if you're in PA, MI, or NC, by all means stick with a Democrat, but in about 40 states, you actually have a lot more voice and potential impact voting Green than you do otherwise.