Remember that the various crystals heat up when they get used a lot. Maybe "Steam engine" just means you use the excess heat to create steam, essentially making your whole propulsion system more efficient.
That's would make a ton of sense, actually. They do that almost everywhere else they use steam today, at least the places Ive seen.
But then the Steam engine is more of a supplemental recovery system, not truly a back-up propulsion. It would need to be run almost any time the Core Crystals are, to improve efficiency with the additional cooling effect. But conversely it could not be run if the Crystals had failed, so it couldnt operate as a backup/redundant system, because it could not operate independently.