I don't think the statement is unfounded at all. There were few child actors at that time and there has always been a strong tradition in the theatre for actresses to play the role of young boys in stage plays with plenty of evidence to be found about this tradition. It has carried on in modern day pantos (in the UK at least), where the principal boy is usually often played by a woman e.g. Aladdin, Cinderella, and, yes, panto versions of Peter Pan. Your experience of performing in Jesus Christ Superstar, while interesting, is not really relevant here.