This article is really sad. It's basically saying "don't ever have a deep friendship where you express vulnerability with anyone who isn't your partner". What makes an emotional affair imo is:

1. Deception/hiding the friendship

2. Turning away from your partner and prioritising the friend more and more to the point your primary relationship suffers

3. That's literally it

I feel like the relationships where emotional affairs occur are ones where partners feel like they can't discuss things openly with each other, including "what if we develop strong friendships with other people" and also where they feel there are certain topics "off limits" with their partner so they turn to someone else to fill that need.