Call it "teaching," "training," "learning," "conditioning," or whatever else you like, but it seems pretty self-evident that people change how they think and feel based on their experiences. Some of those experiences involve acting a certain way and seeing how others react.
Think about it the other way—in terms of negative emotions. A person with anger problems might practice suppressing blowups and taking a moment to try to deal with the anger before moving on. Would you accuse them of being fake-not-angry? Of course not. They're practicing healthy behavior, and over time the practice becomes natural and they don't even have to think about it anymore. They have become a genuinely less angry person by practicing non-angry behaviors.
Practicing gratitude is the same thing, except you're reinforcing a positive emotion rather than suppressing a negative one. You can become a genuinely more thankful person by reminding yourself to express gratitude.
I question any person's ability to truly read the thoughts or emotions of another. At any given moment, a person's thoughts and feelings are very complex and entangled, yet when we interpret their actions we reduce them down to a simple emotion or intention. That oversimplification is always inaccurate, and we should work to develop more complex understandings of other people. So when someone looks like they're faking modesty, they may in fact feel somewhat modest, or at least be faking it for "good" reasons.
There is, of course, a difference between "saying what you're supposed to say" and "trying to be more outwardly grateful," but in reality the lines blur a lot and neither is a bad thing. This may be an unpopular opinion, but when it comes to public discourse, I'll take fake expressions of gratitude over genuine expressions of contempt any day of the week.