The operating system kernel always has full control over the system, how are they suggesting the TPM adds control here? The TPM is a small chip that handles certain crypto operations more securely (especially key management), how does this provide any additional backdoor scenarios?
If Microsoft wanted a backdoor it could easily be added to the OS without a TPM.
Without a TPM, it is possible to detect and remove (or more likely mitigate) a backdoor. With the TPM, even if you know about a backdoor and have a patch you can not apply it without Microsofts blessing. At least, thats my understanding.
One is spying (NSA), another is restrictions on what to install (Microsoft) and some parts discuss the threat from China, manufactures of the TPM chips.