ExpressVPN is the only one afaik that is allowed by the Chinese government. You can assume that it is actually not "private".
When I was in China last April, SSTP worked fine (though long-lived connections tend to become slower over time, and then need a few minutes of cool down before being usable again). Most Chinese people I met were using shadowsocks.
I think his assertion is that as it is "allowed" by China's government that they somehow have access to the keys (or the VPN provider is keeping a buffer of the traffic and has an arrangement to provide it to relevant government agencies upon request).
I have no experience with ExpressVPN, so I can't help you with that.
But in a vlog I've watched on Youtube, the host of that vlog said "Over the last coupe of days, ALL the VPN is been very difficult to use". So, I guess that includes ExpressVPN.
Im in china at the moment using expressvpn (been using it for a year by now) and since about two weeks only three server locations work well (Hong Kong, Tokyo, Los Angeles). Some others work off an on. Before that most locations worked and some of them, Taiwan for example, used to be very fast. Its still usable for streaming and surfing but I'm afraid the end is near. I think sometime in the future one will have to go with shadow socks and or similar protocols/solutions but until then expressvpn is quite convenient (mobile client, router with expressvpn client).
