This is not the case. I've written a Lambda which quite purposefully writes to a different path in the source bucket.

I have a Lambda for processing large files; the first phase is a single Lambda splitting the file into chunks which can be processed within the 60 second limit, and then the second phase is a bunch of the same Lambdas processing those chunks.

(Disclosure: I work with nicksergeant at Localytics.)

This wasn't the case when I was uploading mine, but maybe they've done some sort of smart checking on the execution role (it'd be pretty hard to detect just by looking at the code, I would think).