What is a passive preamp?

A passive pre-amp doesn't amplify the signal. It only attenuates the signal from the source.

A "Passive pre-amp" can come with one or more inputs but the use of the name "passive pre-amp" is inappropriate. Your impression is correct. It is a passive 'line stage', or in the case of a single input its simply an inline volume control/attenuator.

But there are preamps with no gain that are not passive.

There can be active buffering without gain, so the "no-gain" thing is not what defines what's passive or not.

It's not really a preamp without a phono stage. And it can't really be called a preamp if it doesn't pre-amplify. It would really be a passive linestage.

An Amish preamp?

preamp means "before the amplifier."

Those who think a passive preamp shouldn't be called that because it doesn't amplify don't understand the English language.

Active preamps and passive preamps all come "before the amplifier" so the nomenclature is correct.