This rate limiting occurred even when imgur was exempted using disabled_image_download_domains, it was not hotlinking or downloading at all.
Uploads were turned off to promote third party hosting of images, hence the exemption added for popular image sites that my members are using.
This does not explain discourse somehow hitting imgur’s very low threshold. As stated, there have been few images posted already and most have not been hosted on imgur. Offering an upload is one solution but not the one my members will first seek out, and I’d like to avoid using it long-term.