It depends on if the proxy works on the HTTP level or the TCP level.
If it is an HTTP proxy, then in order for the HTTP request to be read, it has to be decoded by the proxy. In other words, they would be able to read the traffic, but the traffic to and from would be encrypted.
If it is a TCP proxy, then the SSL is handled between the client and your server and so the proxy would not be able to decode the traffic without the private certificate.
From the look of it, it appears that they potentially could look at the traffic:
"Currently, we support only HTTP/S protocols. This communication protocol is encrypted using a proprietary algorithm."
It looks like the use cause for this proxy doesn't really match up with "needing to send sensitive data" - it looks like this is more for scrapping site
s and social media that will block requests from the same IP repeatedly hitting the site.