Re: But.. you have the sourcecode right ?
You seem to be making the assumption (or implying) closed source is better in this regard, there is no evidence that it is. Actually, having open source code, means potentially more eyeballs on the code, not less.
As an example to poor coding, Microsoft added the user's Downloads directory to it's 'Clean Up' tool in Windows 10 1809 (the Downloads directory option has now been removed again in a subsequent release).
There was no warning to users that the Download directory had been added to the list of optons on running the new version of the tool (or the fact on realising this 'error' that it had been removed again in subsequent release). Many people run this tool, using the 'Select all' option, thinking it only deleted redundant system files, not user downloaded files. Both are based on trust, trusting those employees not to add malicious code, and for managers to make sure code is checked.
As a coding decision, this proved just as destructive, and anyone (a senior developer) with any knowledge of personal user space v OS temporary files, should have flagged this change immediately, as potentially very destructive and dangerous, in terms of unintentionally* deleting personal files.
* You could argue it resulted in a 'positive' for Microsoft, because it made the case for using One Drive Cloud Storage, but let's not get into conspiracies.
i.e. Closed source is not better by design, and no one should make that assumption, with closed source, the processes to spot malicious code, are just hidden from us, and they clearly don't always work either.