Also this is the first time I've heard of Yammer, while Yahoo is big and old. Is this even a likely occurrence? Have they had or hired many or any yahoo ex employees in the past?

Headline read to me "Small no name startup says they won't hire employees from huge company that's also never heard of them"

Seems all about the publicity.

Agreed. Just reading the headline alone, I wondered to myself how presumptuous he was to assume that many Yahoo employees even wanted to work at Yammer.

Of course it is, but its also a clever strategy to capitalize on engineers who might be feeling poorly about their own company at the moment. As recruiters know, there are a lot of great candidates just under the surface (or 'loose in the socket' as a manager of mine once said). This kind of thing might make them more likely to see employment elsewhere, and like car dealers that offer a free 'umbrella hat' if you will let them get 'first crack at a deal' you can capture some good talent if you are the first company they talk to.

The story helps Yammer reach out to those people and offer them a conversation.

If anyone does go interview there, ask what they they are doing about software patents. One interesting trick would be to let the employee/inventor retain ownership and sign over only a license to the company.