Just my data point:
I have received Amex retention offers within 13 months on different cards:
6/17 -PRG
11/17 - SPG
9/18 - Plat

I can confirm that I received the bonus in each case.

If the rule was “one retention bonus per 13 months across all cards” we’d be seeing a lot more discussion about it. There’s really no reason not to try for a retention offer on any given card once the AF hits.