I agree with this statement, and I think Zynga's actions are at best unethical.
It would be interesting to know the facts from someone in this situation but I also expect that such folks should not be blabbing to folks other than their lawyer.
My interpretation of the story was that they were asking for folks who had stock that was not vested which is to say part of some future vesting pool, to give that up their right to that stock. So if you gave someone 100K shares over 4 years, and they had been there 2 years, 50K was vested and 50k yet to vest, they are asking that you give back the 50k that have yet to vest.
This would be different than Skype's 'clawback' clause, and it would be slightly less onerous than canceling vested but not yet exercised options.
The article also suggests that choices were made based on some measure of value (and implied performance). I have seen folks who are doing ok work, but its not at the level that they are being compensated, that puts you in a tight spot. Few, if any, folks are open to a restructuring of their compensation package in a downward way (which is what Zynga is proposing it would seem). In California at least you simply ask them to leave (and the article suggested that the choice was 'accept this new lower compensation package, or leave, your choice.')
Frankly I think it would be less painful on the company oversall if they just laid off the folks they felt they had made the compensation error on. I don't see anything good coming out of this approach for the company, and I recognize they may think they are being compassionate by not firing people who, except for the size of their option grant, are doing ok.
Google's innovation here is something they call a 'Google Stock Unit' (GSU) (which is not an 'option' it is more like restricted stock) where the ratio of GSU to actual stock is fixed at the time it vests by a perfomance multiplier. That way they can offer a hot shot person 2500 shares of 'restricted' stock (market value of 1.25M$) which vests in four chunks of 625 'units' a year, and if you didn't meet your goal that year your multiplier could be less than 1.0 even 0. So they wouldn't actually have to give it to you if you weren't a hot shot inside of Google. To be fair the multiplier could, in theory, be greater than 1.0 too. The cleverness of that scheme is that the company could 'tune' the compensation of someone dynamically.
I'm guessing Zynga might be wishing they could do something similar for ISO type options.