AITA for attending a party I wasn't directly invited and informing my friend I wasn't invited when he thanked me for my presence?

My good friend milestone surprise birthday party was organized by his wife (40F). Through a mutual friend we have in common, a few weeks ago asked me (43F) if we were attending. I informed her we hadn't received an invitation.

For some context, we had been invited to recent events. In the past, his spouse and I were always friendly towards each other. Engaging in small talk. We live in different cities so I consider more as an acquaintance since I never got to know her. Her husband and I and the rest of the group (who were invited) have known each other for over 20 yrs.

I ended up attending the party as someone guest. The spouse was made aware of my attendance as someone "plus one" a few days prior.

AITA for not engaging with her during the evening and informing my friend (a few days later) that his wife didn't actually invite us?