It still doesn't make sense to base the holiday on the date when one holdout city finally got the word.
As a comparison, at the end of WW2 there were some Japanese units and individual soldiers on smaller islands throughout the Pacific who held out for some time after the war was officially ended. We don't celebrate the end of WW2 on the date when the last Japanese holdout finally surrendered (or was killed), we observe the date when the official document was signed.
I think you may be missing the historical context that people have been observing Juneteenth for over 150 years. And not because the date was significant to the entire country, but because it was very important to them, and to their descendants, and other people who wanted to celebrate with them. That's the kind of grassroots foundation, and tradition, and history, that makes a holiday meaningful, and not just another excuse to take the day off, and have corny sales gimmicks at all the furniture stores. (Not to say that won't happen to Juneteenth eventually. Or maybe, in Texas, it already has. Don't know)
If the U.S. government of 2021 is going to declare a holiday to observe the freeing of the slaves all over the country, I just don't think it makes sense to ignore everything the pre-existing, regional holiday has going for it, just so we can celebrate on a historically correct day from a history book, which no one knows about, because no one's ever celebrated it before. I don't see why we'd pass up that kind of authenticity.
To go back to the Christmas analogy, what if we found out that most Christians didn't celebrate Christmas until, say, 200 AD? And then we found out December 25th was chosen, because that was the day in 50 AD, when some church in an obscure corner of the world found out about Christ's virgin birth, and they've been celebrating Christmas on that date, ever since. The rest of the Christians liked the celebration, so they just joined in on the 25th, and so it is. I get that it's not a perfect comparison, because we don't know the date of Christ's birth, but we can find out the date of the thirteenth amendment abolition. In either case, do we really gain that much by ignoring an established tradition, or trying to move it to some more accurate date on the calendar? I don't think so.