No, I do not think he is odd but just doing his best to be neighborly. He may not have intended to notify your family of the memorial at the very last minute, but he probably had been very busy preparing for the memorial, not to mention grieving. So please cut the guy some slack. Had your husband inquired, he probably would have elaborated more on who the memorial was for. And yes, I would have gone. A neighbor was reaching out at a time of loss, it would have been the least I could do.
At this point, I suggest you stop over with a basket of flowers/plant (or something), apologize for being unable to make it to the memorial and offer your condolences. Then you will likely find out who passed away.
(And no, we don't need to know the deceased personally to attend a memorial. When our friends' parents/siblings/spouse die, we attend the funeral even if we never met them but to show our sympathies to our friend.)
As for the frequency of your 'socialization' with your neighbor: you know how busy and hectic our lives are that time just flies by. Months even years pass by without our realizing it. And yes, we would love to go everywhere we are invited but our lifestyles just don't permit it at times. I am pretty sure you can relate. Just be glad you have a friendly albeit busy neighbor. Your kids don't have to play with the daughter, and it's not like they have the time anyway.