I'm sure this is a meme, but the trust is proving the OS is not tampered with.
Like, if malware was able to inject a malicious windows update URL into the OS, and inject a malicious certificate that gets the OS to trust the malicious updates by the malicious URL.
The signature of the OS would then differ from what the TPM/CPU recorded during OS boot and what the TPM/CPU has hashed during running. This would indicate that the OS has been tampered with.
So the trust in TPM is that the TPM and CPU are working together correctly (which is certified during manufacturing), so that the TPM can then attest that the OS (or software or whatever) hasn't been tampered with.

So yeh, it's MS (or whatever software company) trusting that the software it is interacting with is running as it is intended