For most people I know a cat nap is often a "freebee" where in my case DS2 had his in a sling on me while I got things sorted for DS1. It's a short nap to kind of balance out a day.
The thing with EASY is it isn't meant to be an exact clock watching exercise more that the baby has sleep separated from feeding & that there is a give or take idea of how long a baby of a certain age can stay awake. The other thing about a catnap is it's often a transition in the process of dropping the number of naps in the day. Some cat naps as longer some are shorter, there is no "perfect"
Try not to get caught up in needing it to match the ideals of the book, rather look at what works for you in the big picture. So to me the catnap in the swing is fine.