Diesel does not come with an oxidizer or with a detonator.... Not all BIG backups systems are diesel - though most are and almost all older ones are. New electric cell battery systems (use nat gas for hydrogen source) are becoming common. However all systems I have personally seen have a huge battery of cells with the sole purpose of supplying instant electricity back to the system while the long-term generators power up.
Giving the relatively tiny need for power to keep a printer running, a powerwall will work well, if it does not switch on fast enough (though the published specs seem to say it will switch fast enough) just add a small ups that can run for literally a fraction of a second while the powerwall switches on - the Tesla powerwall is rated at 13.5 kwh - If we take the high power-usage from the OP in post #1 - 4.6kw lets round up to 5kw - then a single powerwall should be able to keep the printer running at full maximum power for 2.5 hours... quite reasonable for a BIG UPS...... and only about $10k... and with net metering it can generate income.... you can also find 35kw backup generators which claim to be fast enough for electronics in the $6k range - but these will operate at a total loss.