The source code being downloaded, compiled and run or a script being download and run would be a as much a security incident as what happened.
In this context (Chromium on Debian) having a closed source binary downloaded and executed is an additional problem to the security incident and that's the reason it is mentioned in the statement. There are two problems conflated in the same sentence:
1. A binary was downloaded and executed without explicit user intervention or consent.
2. A closed source binary was downloaded and executed by a primarily free and open source software in a free and open source distribution without explicit user intervention or consent.
So, answering the questions, having the source available would not make it ok but being closed source in this context is a problem on its own.