Is MySQL/MariaDB getting a better IOPS deal than Postgres in RDS?
As I read that page, "Provisioned IOPS works with an I/O request size of 32 KB. An I/O request smaller than 32 KB is handled as one I/O; for example, 1000 16 KB I/O requests are treated the same as 1000 32 KB requests.", your conclusion is correct.

MySQL has an additional advantage: innodb_flush_neighbors.  If on, it will try to combine adjacent blocks together, thereby cutting down the number of IOPs.  (This is designed to cut down on head seeks on hard drives, but counterproductive on SSDs since there is no seek time.)

(I don't know if Postgres has a similar thing.)