I think you're missing the point being made. GP isn't saying these schemes require a central server.
The point they're making is that PoET assumes that Intel SGX actually provides the properties that Intel claims (namely that they will never produce attestation certificates for code not run in SGX). Thus there is a single party which you are trusting to provide these properties and not attack your system. If you're okay with trusting a single party then the system is equivalent (in terms of security) to having that single trusted party run a server which emits timestamps.
RRR suffers from basically the same problem -- you're trusting Intel to not produce endless fraudulent identities and thus always be chosen as the oldest miner.
The point they're making is that PoET assumes that Intel SGX actually provides the properties that Intel claims (namely that they will never produce attestation certificates for code not run in SGX). Thus there is a single party which you are trusting to provide these properties and not attack your system. If you're okay with trusting a single party then the system is equivalent (in terms of security) to having that single trusted party run a server which emits timestamps.
RRR suffers from basically the same problem -- you're trusting Intel to not produce endless fraudulent identities and thus always be chosen as the oldest miner.