I'm surprised you're so calm about it. I would have been furious!
I was annoyed when it happened, but it was a few years ago, and I try not to dwell on negative experiences. Usually about a day after something happens, I process it and move on.
Also, it tangentially led me to go back to doing technical work, which I much prefer. Having all my responsibilities as a manager removed forced me to decide what I really wanted to do at that point in my career. On the whole, I'd say it worked out for the best.
The person I worked with was apparently a really really good friend of the person I was interviewing, so I assume it just came up in conversation, like "Hey, do you know so-and-so. They're applying for a job here."
Amusingly, the guy actually did speak well of me to his friend, he just then proceeded to notify my boss that I was interviewing at another company.