I go with what William Porcher Miles wrote at the time and said at the time not people 60 or more years later. He was of French Hugenot descent so the Scottish Cross had no meaning to him whatsoever! Miles was also well versed in heraldry and did not call it a St. Andrews Cross - he called it a saltire. During most of the war it was called the Southern Cross by Confederates. The nonsense about the Celtic/Scottish meaning was a 20th Century creation by people who simply did not do their research properly.
I found my notes of all of the flag submissions to the Committee on Flag and Seal. As follows:
Flags based on the Stars and Stripes - 99 submissions
Unique designs - 50 submissions
saltire/Southern cross design - 6 submissions
Latin/St. George's cross - 33 submissions
The above pretty much shoots down the myth of the Scotish/Celtic thing too, for if there was so much of that in the South there would have been lots more of them submitted by the Southern people for their national flag. Since the South was mostly of English descent the figures above are not surprising at all.
You are free to believe what you wish; I go with the facts.