As Anti-Interdiction it is part of the “chain of trust”, as Purism calls it, it’s only important (or useful), if such a chain exists at all. So, for vendors that care less about fully free (auditable) firmware, and without something comparable to PureBoot, such an offering makes less sense.
It starts with the hardware that, ideally, would be fully open/free.
Ideally, the hardware is assembled by somebody you trust. In the case of the Librem 5 USA, that would be Purism, not a Chinese contractor.
Next best is, having at least only free firmware. Purism is close to that, at least for the important components.
On top, Purism has PureBoot, meant to signal you malicious modifications.
But all of that doesn’t help if a high profile attacker can make modifications before PureBoot is under your control.
Further reading:
What is TPM and do I need it?
PureBoot Best Practices
Announcing the PureBoot Bundle: Tamper-evident Firmware from the Factory
Protecting the Digital Supply Chain
Anti-interdiction Services
Anti-interdiction Update: Six Month Retrospective