I thought unity gain means exactly +1 and nothing else?
Gain of -1 doesn't actually require a unity gain stable amp, but one stable at 2 or more. It's noise gain that matters.
Yes, that's true. It's the noise gain which matters. An op-amp which is not unity gain stable can easily be used as a buffer, by setting the noise gain to a high enough value. It does increase noise, but this can be mitigate somewhat by only having a high noise gain, at high frequencies, which cause oscillation. Noise is also less of an issue for a buffer, as the signal level is generally very high.
An application of this is connecting two audio amplifier ICs together to make a bridged output. A gain of -1 is required to make one side inverting, but many audio amplifier ICs need a much higher gain to be stable. The solution is to set the noise gain high enough to be stable, even through the signal gain is unity.
See the thread linked below for more information.
https://www.eevblog.com/forum/beginners/op-amp-spec-noise-gain-configuration/msg2401983/#msg2401983