I don't know but IB runs a lot of websites and I think they prefer to use a standardized platform for all, without a lot of customization overhead for what's a relatively small user base (compared to say a Ford F150 forum). I suspect their setup is tailored to avoid known scam/spam sources, which may include some VPN's. That's presumably largely automated, and might throw out some baby with the bathwater.

I've never had issues with Nord VPN either, so there's that.