He claimed in interviews that he had not decided what to say until after the landing. Then his brother later claimed Neil told him what he was gonna say? I dunno. A bunch of pedantry over the “A” doesn’t make sense to me. It’s sort of implied. You’ll never hear it. Try speaking the line, it’s not really there depending accent. Why it became a talking point is strange.
I'm 60 (OMG!!! I'm officially OLD!) so I've listened to many interviews over the decades with Armstrong. Neil thought long and hard what the very first words spoken from the surface of the moon would be, and it was supposed to be "for a man" to contrast with "mandkind." If you recall, after Neil descended the ladder, he stood on the Lander's round pad on the end of the landing strut, so he wasn't technically on the Moon yet. He hesitated on that pad for a couple of minutes as he made sure his suit and other equipment was OK before he took one step off of the pad and onto Lunar soil and made his speech
However, when you listen to the recording of his speech, you can hear after he says "that's one small step for man..." that he hesitates, probably saying to himself "darn it, I messed up my line." Then he says "one..." then hesitates again, probably thinking "should I start over?" Then he finishes very quickly with "giant leap for mandkind," with I'm sure the thought of "forget it, it's over now."
You're right, it's a small thing, and I've read articles in the past where Neil was OK with the "a" being in parentheses when written, as in "That's one small step for [a] man, one giant leap for Mandkind." I've also read articles that claimed Neil did say the "a" but the VX (voice operated transmit) circuit on his suit radio clipped it off. No, it didn't.
I just don't like to see revisionist history on things that I've lived through.