Attachment Parenting means you strive to meet her needs, even when they are not convenient for you. Are you meeting her needs? She doesn't seem like she needs you if she is happily playing.
Part of the reason I sometimes struggled with "meet their needs" is that at this age it becomes trickier to distinguish between "needs" and "wants." As a newborn, if my son needed something, he cried. My boys are now 4 and 7, so they will sometimes cry if they don't get what they "want" and not just because they "need" something.
You are not neglecting her by letting her play by herself. She is happy, she is learning, she is discovering more and more about her world, she is learning to play without Mommy, and that is a good thing. You would only be neglecting her if you were ignoring her needs.
And don't feel bad if you do occasionally ignore a need. Sometimes you have to use the bathroom and then take care of her. And that's ok. Just make sure you take care of that need as soon as you can :-)
ETA - Attachment Parenting does not mean that you have "strive to make EVERY SINGLE MOMENT with her an opportunity for attachment." That is so NOT what Attachment Parenting is. It really means striving to meet the needs of your child. You are about to hit an age where you will have to distinguish between "needs" and "wants," which means asking yourself, "What does my child really NEED from me right now?" Sometimes you have to remind yourself that your child does not need to stay and the park and play, even if she wants to, because you know she really needs to take a nap. It's not about doing what they think they need, it's about doing what you know (or are pretty sure, since we're only human) they need.
Sorry, personal pet peeve. I hate when people think Attachment Parenting means doing whatever the child wants. If we did that, we'd all be raising spoiled brats!
Oh, and I didn't mean that you think that, just that I often hear from other moms who think that.