Hello,
I believe [NEXUS-23790] APT proxies: "distribution" field is required but should not be - Sonatype JIRA speaks to what you found. From that ticket, it looks like it’s not needed for proxy but presumably needed for hosted in the current implementation. I am not clear if the bug is that it should be needed for proxy or that it shouldn’t show as a (required) field for proxy. But hope that helps.
-Joe