If they had increased prices in 2022 (or at least announced in 2022), then I could see some kind of correlation, but give it was 1.5-2 years after, I doubt there is a connection.
> i would expect aws needs a year or two from when they decide to charge for something new just to work out the details
The price had already dropped, and was continuing to fall, when they announced the change, so if rising acquisition cost was the primary reason for adding the IPv4 charge, it had already went away.
I think AWS has looked at a utilization graph and sees a time their current pool is get used up at current rates and doesn't want to go through the hassle of acquiring more IPv4 addresses, regardless of cost (even if it is "cheap").
I also think that they have statistic for their www.Amazon.com storefront, and maybe are seeing a good proportion from IPv6 and so figure that there's a 'critical mass' (especially mobile).
Now IPv4 prices are returning to pre-Covid long-term trends. But of course Amazon won't reflect that in their pricing table.