If we are talking about bigger projects with hundreds of thousands or millions of downloads, than this may be true. But smal scale projects have so few people actively looking through them that even to automatic scan done by the playstore has a higher chance of catching malware. It doesn't even have to be bad intent, two years ago there was a virus propagating trough the Java class files in minecraft mods which reached the PCs of quite a few devs before it was caught.
I don't dislike FOSS, a lot of the apps I use come straight from github, but all this talk about them beeing constantly monitored by third parties is just wishful thinking.