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
“an 1000”
‘1,000-year-old’ is an adjective describing the stone trophy.
I would go by the NOUN which is the subject of the sentence, in this case.
I was trying to recall a case where the number would be the noun/subject, and what I would use then . . .
Give me Liberty or give me a ticket to Florida!.................