Presumably they are talking about a VPN service hosted somewhere outside of your ISP's network, so your ISP could only throttle traffic between you and your VPN

Presumably they are talking about a VPN service hosted somewhere outside of your ISP's network, so your ISP could only throttle traffic between you and your VPN service but not between the VPN service and the content provider. The VPN's ISP or the content provider's ISP or any transit provider in the middle could throttle between the VPN service and the content provider, but there is far, far more competition among ISP's suitable for server farms and transit providers than there is among residential ISP's so they are much less likely to be able to get away unreasonable throttling.