Seriously, how do comic characters manage to be condescending and drink their beverages at the same time? Same thing I regularly see on Two Guys and Guy. It's a skill that would greatly enhance my own moments of condescendence (given that most of them happen with a coffee mug handy)
It's the eyes, and the eyebrows. Step one: Make scathing rebuttal. Must be short and succinct. Step two: Sip your beverage with slightly narrowed eyes, keeping eye contact. Raised brow(s) for extra impact. This communicates to your target your complete self-assurance in putting them in their place. You are basically indicating to them that what you have said is so beyond reproach that they have no possible reasonable counter-point, and the discussion has ended. In short, it's like a mic drop. Except that dropping a cup of coffee would be much messier.
As far as "Marten is no longer spineless" goes.. It is true, to a point. He is not feeling secure in himself and his place in the world. He has a girlfriend who is his equal and they can verbally spar without hurt feelings because if something does bother one, they talk it out. One of them doesn't fly off the handle. Marten has a history of surrounding himself with strong-willed women. Being raised by Veronica no doubt plays part in that. In his relationships with Faye, Dora and Padma he was always walking on eggshells. Saying the wrong thing to Faye or Dora has blown up in his face many timed. Padma he didn't want to deal with the issues head on, because he knew it was a temporary relationship from the start, and he hated that he got so invested so quickly knowing how it was going to end.
Now Faye and Marten have settled their relationship as best friends and nothing more. There is still tension there at times. But Marten knows that saying the wrong thing won't drive her away. Something he's always been afraid of. When one of them pushes the other hard, they both know that it's because they are concerned and care for each other. Even if they don't feel that in the moment, they know it's not going to break their relationship. His relationship with Dora has become better as just friends as well. Both of them were in a poor state of mental health when they were dating, and have taken steps to address that since the break up. And in fair part, because of the break up. Dora has been working on her paranoia and control issues. Marten has stopped using avoidance as his main means of dealing with conflicts with others. Which is where the 'spineless Marten' meme comes from.