It is what I have been saying from the beginning. That is fixed, constant ... that phase does not vary with the frequency ->linear, FLAT, ideally 0º in ALL audio band.
It has always been that way and a new amplification technology should respect it.
Imagine an audio chain in which the source, the preamp and the amplifier each vary the phase with the frequency ... We have to eliminate variables from the equation and not to add a new one.
It is impossible to achieve 0 degrees of phase shift in (through) a real system -- there is always some delay unless you have infinite bandwidth. If you delay each and every frequency equally, the signal will arrive all nicely lined up in time, but that requires linear phase.