Unstrained yogurt has the exact same ratio of water/protein/fat as the milk that it was made from. The amount of sugar will be different as some of the sugar was converted to acid during the fermentation.
This means that yogurt can be substituted 1 to 1 for milk, and sometimes other liquids, in baking. Know that the pH of the batter will be lower and if that will be a problem then add a little more baking soda to compensate for it. The lost sugar won't usually adversely affect a recipe because they usually have plenty of sugar added.
Quickbreads don't usually have a lot of added liquid and adding a significant amount of yogurt would throw off the batter. It could still be good but it wouldn't be the same thing.
To mitigate that problem your recipe uses Greek yogurt. Greek yogurt has been strained which simply means that the yogurt was stirred to break up the protein mesh and some of the whey was allowed to drain off. The whey will be mostly water but will also contain some sugar, lactic acid, and riboflavin. Even with the whey drained off Greek yogurt is still mostly water. This water, however, is locked up in the protein mesh. Where normal yogurt will weep whey when cut, Greek yogurt will not (at least not so easily).
The yogurt will regardless add some liquid, a fair bit of protein and potentially fat, and some acid. The acid and fat will add some wonderful flavor while the protein and fat will interfere with gluten development. In quickbreads you don't need gluten so this is a good thing.
To compensate for the additional acid your recipe includes a little extra soda along with its baking powder.
Yes you can add strained yogurt to a quickbread for flavor. Be careful not to add so much that the added water will throw things off and remember to add the extra soda to compensate.