Computer cases are electrically insulated.

As any radio amateur will tell you, PC shielding in the wild is generally poor. Plus the vast majority of PC RFI is radiated via the connected cables ( be that USB or the power cable ) or the monitor which form broad-spectrum antennas.

A PC and its appendages is a horrendous radio squawk-box and should be kept away from anything RFI-sensitive. The FCC 'must not cause harmful interference' clause is pretty worthless.

Thunderbolt is PCIe.

Not quite: https://twitter.com/whitequark/status/1097777102563074048

"Thunderbolt, uh, does not expose PCIe lanes directly. Thunderbolt is an MPLS-like packet switching network that can encapsulate PCIe TLPs over a PHY and MAC without a spec, chips without documentation, and software with barely any support."

Thunderbolt encapsulates PCIe frames. It's not PCIe any more than say ethernet is UDP.