I highly doubt that this is true. HTTPS POST data is encrypted just like any other HTTPS data. But, if one is using CloudFlare or any other proxy, by nature, that data needs to be decrypted and then encrypted again on it's way to the destination server. So, yes, of course, each and every proxy can see the data. And, no, HTTPS POST data can't be snooped over the wire otherwise.
If CloudFlare, any "relay" were compromised, all traffic through that would be compromised. That's everything. All your bank details, full access, it's all right there.
The selling point of SSL is nobody can read what you do between you and the final destination, except when the developers of that destination enlist CloudFlare who have a root certificate that allows them to intercept everything whether you realise or not.