They are saying developers are the source of bugs (mistakes?), but not in the way you are suggesting. Developers are the source of bugs in that they write the code which includes the bug, and so it is not particularly surprising that you can read biometrics that indicate when developers are likely to produce code with bugs.
For example, if the developer hasn't slept in two days and so has saggy eyes and wildly drifting eye movement then that's a pretty good indicator that there will be some bugs, and indeed the developer is the source of the bugs because they are the source of the code.
Of course, the manager standing over their shoulder with an unreasonable product release deadline and the threat of job loss is probably responsible for the bugs in any reasonable sense.
If someone is driving a car wearing a blindfold and crashes, they crashed the car. The person who put the blindfold on, held the gun to their head, and said 'drive' is probably responsible for the crash by any reasonable definition.
In summary: I don't necessarily think it is offensive to say that bugs are coded by developers, because they are. However, it is offensive to say that they are responsible for the bugs without taking into account the broader context in which they are working (and indeed, saying they are responsible for the bugs still doesn't necessarily mean that they are in some way wrong or deficient for entering a bug. People - even brilliant people - can and do make mistakes, and that is why review processes do (or should) exist.