that reads more like a statement rather than a question. Regardless of location source, whether google sanctioned or not, and if one has decided to install things from places other than google (arguably the 3rd party check box is the type of thing that makes android a google project versus pure OSS, or in the same respects is what differentiates chrome from chromium), etc, etc, when you install an application, you will be prompted. This prompt shares similarity with, for example, running a downloaded executable on windows and being prompted before actually running.
Having 3rd party sources disabled is not a safety net for avoiding issues, that's simply what makes android a google product.