I don't make that assumption, I worked on developing TPM modules myself in the 90s at research labs, and our prototypes had even more anti-tampering than so far revealed about Secure Enclave/Trustzone: we had micro-wire-meshes in the packaging to self-destruct on drilling or decapping, we had anti-ultrasonic and anti-TEMPEST shielding. I'm pretty familiar.

The point is that state actors have vast resources to pull off these attacks. The NSA intercepted hardware in the supply chain to implant attacks as documented by Snowden. Stuxnet was a super-elaborate attack on the physical resources of the Iranian nuclear program, which was obviously carried out with supply chain vendors like Siemens. Apple uses Samsung as a supplier, and the US government has very high level security arrangements with the South Koreans, so how do we know the chips haven't been compromised even before they arrive at Foxconn for assembly?

Here's an example of a TPM module being decapped and hacked at Blackhat: https://redmondmag.com/articles/2010/02/03/black-hat-enginee...

Attacks have been shown using silicon doping, security fuse cutting, etc.

If the NSA really wanted to crack the Secure Enclave, I have very little doubt about their ability to carry it out.