A choir doesn't need equal temperament because they have a continuous tuning versus a piano. A string quartet (and a trombone!) can do the same thing, they can adjust the tuning based on the instrument, the octave, and what everyone else is playing because they all have continuous tuning versus discrete tuning (I am a trombone player, so my individual instrument has its own quirks at different partials, but I can use the tuning slide to adjust and be in tune exactly).