It IS a cool ad; I wouldn't be surprised if it was an alternate cover illustration for 252. But it's not a first appearance because the character is not introduced into continuity, into the story universe that is, in this case, DC.
It's a neat thing, maybe even worth a little more than surrounding "run" books for this reason (I bought one because of it) but it's not her first appearance in that it is not her debut into the serial illustrated fiction that is a comic book "universe." That's the standard.
I have a My Greatest Adventure #79 in my collection because it mentions on the letters page that the next issue introduces "The Legion of the Strange," and if I one day sell it I will point out this fact because it is the reason it might be of interest to Doom Patrol collectors, but I do not expect anyone to pay for it more than a slight fraction of what #80 goes for.
Yes, I'm aware of Malibu Sun #13, I have a couple somewhere; I'm happy to be able to sell them for much more than the cost of a Spawn #1, but a large group of Spawn completists driving a relatively scarce (compared to Spawn 1) preview does not change the rule for the hobby. It's a cool anomaly in the market, but it's not driving DD 115 past IH 181 anytime soon.