ICE test train reaches speeds of up to 405.0 km/h

The ICEs are already plenty fast, the issue is they share rail with the much slower and less reliable RBs. Any delay cascades and you can't just make ICEs go faster to catch up.

On a €/(avg. ICE speed) basis likely makes more sense to invest directly in RB.

Yea. Who cares if you can hit 405 km/h if you are just going to get stuck behind a train carrying goods for 2 hours unable to move.

While DB is obviously involved, this test train included cars from a new design that Siemens primarily is aiming at the export market (E.g. US Brightline West, various projects in Asia, ...).