if you're actually interested in making political change and you don't like Democrats, possibly the worst alternative avenue you can channel that through is the Greens. the Greens, and i cannot stress this enough, are not a serious left-wing alternative, and you cannot make them be one by protest voting. they aren't even on the same page as to what "left" means--there is a principled ecosocialist wing, but also a massive crank wing that pollutes the party--and most of their state parties are completely atrophied. your time is almost certainly better spent joining an actual socialist organization like Socialist Alternative or DSA, or just foregoing electoral action entirely. look up how to organize your workplace, or volunteer with Food Not Bombs if you're interested in the second half.

First of all, they specifically said general election. You're making an assumption that they voted Biden in the primary.
Second, a vote for the green party was basically a vote for Trump in the last 2 general elections.
Third, in reference to your comment slightly higher up on the thread, Biden did what he could to cancel 20k in student debt. The reason it hasn't happened is 100% due to republican lawsuits.
But you just go ahead and keep feeling betrayed.