What the NSA was looking for was a mistake in implementing the encryption, or rather, for a significantly faster/easier way to find and exploit such a mistake, if one exists.

They specifically asked for the code for this purpose. If you made no mistakes, your action did not help them nor hurt your customers. But if you have made a mistake that you're not yet aware, you took a conscious step to make it easier for the NSA to exploit that.

While the availability of source code does not make a difference in theory, this falls apart once you realize that even though the NSA has lots of resources, those resources aren't infinite.