It wasn't really my intention to get into a philosophical debate based on this silly little anecdote about Radcliffe, but here is a quick rundown.

Society has traditionally seen humility as a virtue because it is the counter to all the negative aspects of pride. Pride is related to arrogance, to an egocentric worldview, and to a reluctance or even an inability to be self-critical, to be honest about one's flaws, weaknesses, and limits.

Humility is venerated because it embodies a spirit of working to avoid all those things--to be realistic, introspective, self-critical, honest, modest, and respectful, and not to see oneself as more important or valuable than they are in the grand scheme of the world.

Your confusion is coming from the fact that you are equating pride and self-esteem. They aren't the same thing. People can be prideful and have very high self-esteem or very low self-esteem, and people can be humble and have very high or low self-esteem as well. That's why:

To say pride is a sin is to say self-esteem is not a moral value.

Is not true.

You are imagining that everyone who's humble is depressed, or believes they can't achieve or accomplish anything, or denies their own abilities and strengths, or that their life isn't good or interesting or worth celebrating. But that's not humility. Humility is just a balanced level of perspective and attitude of respect for oneself and others.

And as an afterthought, to address the point you made about the OP's picture:

but being proud of an achievement and using that as your reasoning for not giving a fuck what people think of something as stupid as your height seems perfectly valid and moral.

The counter to this that's in the vein of everything I said is that someone who was humble and at peace with themself wouldn't need to use achievements to make themselves feel secure from the judgments or concerns of others. They'd just not be burdened by the fact that they were short at all, they'd accept it as part of themselves rather than needing to accomplish things to "compensate" if you will. They'd achieve things for the merit or utility of those achievements, not for any other reason.