Maybe over the capacity allocated for VPN originating traffic? I would never interpret a user-facing error message like that on a public site to be a reliable claim of what's actually going on in the underlying infrastructure. For any service.
That's very charitable. I can't imagine that they would segregate bandwidth like that. I've never had Imgur work on VPN no matter the endpoint or time of day or week.
While using a VPN, they will say "Imgur is temporarily over capacity. Please try again later." But if you don't use a VPN, then it magically works.