It's what he meant to say but his line got messed up. Some claim the recording shows he actually got it right but it went down into history with the misquote. In either case you'll note I did not make it a "quote" but a quip instead.
Yes, I have heard all the stories about how he supposedly messed up, but I had the recording back in the day and have listened to subsequent recordings. But yes, I did note that you did not put the remark in quotes. That was heads-up on your part. :)
Some, including Armstrong, blamed the VOX circuit on his radio with cutting off the word, but listening to the original transmission, he says:
"That's one small step for man, one"
Then he hesitates, and you can just imagine him think to himself "did I just screw up the one line I've been practicing in front of the mirror for the last five years?
Then he quickly finishes:
"giant leap for mankind."