Haven't seen that ad, but I've been to NYC dozens of times and never saw any vintage cars there, let alone a '67 Mustang. Not exactly what you want to drive in that horrendous traffic.
I agree that there's no way the GM marketing people didn't see the Mustang in their ad. Every pixel in an ad is scrutinized/colorized/modified before it's aired.
But, as for their mindset of why to include it, instead of a vintage Camaro/Corvette, I have no idea. (Unless someone in authority is a Ford lover!)
And, yes, Cadillac moved its HQ to NYC.