Can developers choose to use an infrastructure provider other than nbn?

Yes. Nothing in the Telecommunications Infrastructure in New Developments Policy prevents a developer from selecting any provider to supply infrastructure in, or to service, a development. Any provider is free to comply with such a request, but is not required to do so, other than nbn and Telstra in the separate circumstances applying to each of them as "providers of last resort" as described in the policy.
Read more about the government policy for new developments