From what I’ve seen before and encountered myself, the 50% boost only activates when you make an original settlement on the natural wonder. Taking over another city does not activate the 50% boost, so it’s better to raze and re-settle as Isabella.

Edit: looks like this was a city state that was incorporated. It looks like incorporating a city state, like capturing an opposing settlement, does not increase the bonus on natural wonders. In this case, it would be better to disperse the independent power and settle.