If you say the number 1000 correctly, one thousand, the first word starts with a vowel, hence the “an”.
I can say that numeric correctly, and still prefix with 'a' if I so desire.
The rule you pointed out is something I have never known until now, so it cannot be important. These language rules are entirely arbitrary and some people want them dictatorially enforced.
Give me Liberty or give me death. ;-D