I'm thinking, if he plays with them regularly, they may have just put a party together at the last minute. Yes, it is strange, however, I've had this happen to one of my children, too, and in my case, it really was a last minute decision for the family to even throw a party (my daughter was asked the morning of the event!) Especially given the fact that it is a "5 hour party on a Friday night"? Sounds like they are scrambling. I would be curious if this is ALL children or if it would be an adult family party, too. I've had "sleepless sleepovers" on a Friday for 8 year olds but, even then, it was a 4 hour party.
Even if your child was invited at the last minute - would you want to know that??? And, even if you did, you wouldn't be sharing that info with your son or, "just knowing the truth" could alter your opinion of your son's friends and their family. Sometimes ignorance is bliss, especially, if this is a neighbor and your son hangs with them on a regular basis. Some things just aren't worth knowing. However, I can see why it would raise an eyebrow, with you.
Good luck.