Ahh .. I bet it must've been quite the performance indeed!

Usually choristers and vocalists in general, would require the assistance of a third party instrument to tune their voices before a performance, in order for them to sing on the right key. Much like you said, a keyboard can be used for this; or in some cases, something known as a tuning fork can be used also.

However, in some rare cases, some vocal soloists wouldn't always require keyboards or tuning forks to tune their voices - i.e if they have perfect pitch or an excellent grasp of relative pitch.

In all, that's just the magic, science, and beauty of our brains in relation to music! Might seem quite impressive to an outsider but in reality, these are just fundamental basic principles 😄