Some components of our apps are open-source, but the main apps are not. This is to protect you and us from unscrupulous counterfeiters who add malware and ads to open-source apps.
I am not following these claims carefully, but I have seen tons of copies of Lawnchair in the Playstore.
Another recent event that comes to my mind is the Simple apps, which AFAIK they always were open source? But that didn't matter until it got sold and then Fossify was the non shit version of it (the positive side of open source).
Vapid argument: You can always check it yourself. That is the benefit of open source: you can inspect the source code and see if it's malicious or not. You would usually know if an open source project is malevolent or not; if you still have some dubiety, you can always deploy it yourselves.