Op, honestly you have done nothing wrong.
I have 4x DCs, aged nursery up to yr 6. There have been many parties. There have been basically no dramas over parties that I can remember. The parents who make the dramas, are the ones who end up with the kids with friendship issues.
Sometimes odd situations arise... Maybe it's you who goes on a play date with your child, and then you realise the mum didn't actually want you there, just your child (yep, that was me). Maybe your child insists they're invited to a party, but no invitation, so you have to approach the other mum somehow, trying to make it clear you don't mind either way, you're just trying to find out what happened (also me).
Please try not to let these situations bother you. Sometimes they are a little awkward. Sometimes you get it wrong, sometimes the other parent does. Kindness, good grace, a bit of humour, and being willing to shrug and move on will stand you in good stead. After all we are all just making it up as we go along!
In this case, I would find it a bit strange, but she was invited anyway, so simply provide the details and forget it.
We have given out invitations in the playground, and my children have taken them in and the TA has put them in book bags. Never invited the whole class. Have also had situations where more people needed inviting as loads couldn't come. We have also invited an extra child on occasion when one of mine told me someone was sad. We have had uninvited siblings (smiled, scraped together a party bag, offered food and cake). Heck, we have twice invited children labelled as unsavoury, or bullies by the other parents who questioned whether they were coming. They all behaved fine and said thanks at the end. Hopefully we have not offended people, if so I have never been aware and my kids still have friends.
Don't sweat the small stuff, it's a long time until the party phase is over.