Why would my former colleagues acknowledge me for a nonexistent contribution?
I see two possible reasons for their behaviour. Others options are possible.
- They are using your name to reduce the chances of desk-rejection. I have heard that a well-known expert in the acknowledgements can be seen as a positive point for some editors.
- They copy-paste the acknowledgements of a previous manuscript and forgot to delete you.
A possibility which have not been mentioned: If you are in the acknowledgements section, you will not be asked to review a paper. Especially within narrow sub-fields, this can actually mean something.
Another option I see is that they simply discussed something with you while you were still there, and they got something useful out of it that made its way into the paper. And moreover while his happened they didn't explicitly tell you "our current discussion is related to this specific project, by continuing to discuss with us you agree to have your name in the acknowledgements section of the paper, please sign this affidavit stating that you understand".