This older comment explains how ECH works.

ECH is technically unrelated to DoH, ECH is a HTTP extension not a DNS extension. But it uses the DoH encryption because it can’t use the HTTP encryption because of the chicken-and-egg problem explained in that comment, so… it basically latched onto DoH as a solution and in doing that tied the two together.

And to answer your question, DoH is usable on its own without ECH because ECH is not needed for DNS. But ECH is strongly desirable for HTTP, and it also requires DoH, so that’s why Mozilla for example activated then as a package deal in Firefox (both or neither).