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

Honestly, the user is the biggest security risk in the first place. People run all kinds of malware and put their passwords into phishing sites all the time. One thing a TPM is used for is secure boot, which prevents malware from inserting its own bootloader to take over the OS.

But who is supposed to trust whom?

12 years old and still relevant:
https://www.youtube.com/watch?v=s7WDbnHlc1E

Reminds me of the clipper chip