I guess this expansion used dynamic RAM with on-board refresh logic, but the page isn’t clear (to me) on that.
It's a little bit of both. The RAM on the expansion card is an ISSI pseudo-SRAM:
https://www.digikey.com/en/products/detail/issi-integrated-s...
In short, it's DRAM on the inside, but all of the refresh logic is internal, so it behaves as if it were static RAM.
You're right, the card uses PSRAM because non-synchronous SRAM chips basically aren't available new any more. I didn't want to have to troll the depths of (semi-questionable) new old stock parts sites and went with new chips.
This design was assisted by my good friend techknight, original designer of the 8 megabyte RAM card for the mac portable.
AS6C6416 4Mx16. Someone even uses it raw on 5V bus with no level translation, just tiny Lattice LC4064V cpld for address decoding. https://www.amigalove.com/viewtopic.php?f=6&t=1689 https://hackaday.com/2021/01/08/the-amiga-1000-still-receivi...