Using DB/2 surely makes a lot of sense for integration with the rest of their IBM tech but also something the new system could presumably talk to too. I would guess that it performs ok at the kind of ‘single big OLTP database server’ tasks it was meant for. Notably IBM allows you to publish benchmarks for their databases (so long as you provide detailed methodology and take steps to tune it correctly) whereas Oracle and SQLServer do not allow publishing benchmarks without permission. Though one could argue this is just IBM feeling like benchmark performance wouldn’t matter much to their customers.

The IBM stuff is pure legacy. Of course this would all perform much better with modern tools, like Postgres.