You answered your own concern. The entire article, and indeed history, illustrate why we have a 2-party system and why it actually works.

All good legislation was passed because formerly unaligned voters aligned with one or the other party. A good example is the progressive movement in the late 1800s to early 1900s.

Imagine the weight at the ballot box if the Green Party integrated into the Democratic Party!

All politics require compromise. In countries with multiple parties (generally parliamentary systems) a government cannot be formed without a majority win. If no single party gets a majority then two or more parties join a coalition to achieve majority. Doing so means each of the coalition parties expect to get something by joining but also give up something as well. So there really is no reason why Greens couldn’t become a “green wing" of the Democratic Party to advance ttheirideals and policies.