Catholic monks, eg Benedictines or Carthusians, often sing very well. According to stories I've read some sing not so well, but most are amazing it seems. They don't do warm ups before singing the Liturgy of the Hours but still they sing very well. How do they manage this without warmup exercises?
No-one NEEDS warmups. Warmups are good. They aren't essential. We've all had to 'go in cold' whether to a musical or any other physical activity, and we came through OK, didn't we?
When you do something every day, and it's well within your capabilities, a warmup may well not even make any perceptable difference.