I think that they are pulling your leg about not adding more water, especially if the original pond was small.

We don't have all the information about the dimensions of the old and new sections, but just for a fun math problem, we can make some assumptions. You can use the example to see what could have happened when you know the actual dimensions.

New pond section: 10 ft deep by 3 feet wide by 3 feet long (volume is 90 ft-cubed of water needed to fill it). I am assuming the new smaller part is like this to give some example calculations.

Old pond section, it doesn't matter how deep the pond is, it only matters that it lost 1 foot depth of water, but it would have needed to lose approximately 90 ft-cubed which went to fill in the new section.

With losing 1 ft deep of water, then the length and width could be any combination of size that is equal to or close to 90 (such as 30x3, 45x2, 15x6, 10x9). That would give it 90 ft-cubed of water loss.

So if the old pond section is large enough and the small pond section is small enough, then yes the water could fill in the new section and seem to have not affected the original section too terribly much.