Actually, a slight correction. The Visa, MC, and Amex were never considered separate products. Various other things have changed which have affected Citi AA churnability, but I've been churning AA cards for close to 10 years now, and never did it matter to any of the rules (that have changed multiple times over time) whether I was getting 2 Visas or 2 MCs or 1 Visa and 1 MC or whatever. Even the Amex was not considered a separate product.

It never mattered what the product was; what mattered is what the terms of the offer were. If it was an offer that allowed "fast" churning, then "fast" churning worked. if it was an offer that was officially restricted to "first time" cardholders, then "fast" churning didn't work. (Please note: Sometimes, for certain offers, the published T&Cs were incorrect, or at least contradictory between landing pages and application pages.) The difference between "fast" churning and "slow" (12 to 18 to 26 months) churning was time, not product. After the required time (which as I said changed from offer to offer), it never mattered whether your previous card was the same (Visa vs MC vs Amex) or not. (And, also, Citi never cared whether you had cancelled the previous card or not, just how long ago you'd last applied for it.)

The only suspicion (not necessarily ever corroborated, because it didn't last for more than one or two specific offers) was that a Gold card (lower tier) may have been considered separate for a brief moment in time than a Platinum or Signature (higher tier) card. But that still may have simply been the individual offers...

The only real distinction Citi makes is between personal AA cards and business AA cards. Those always were and still are totally separate.