Not completely true, as it depends on the architecture involved. Everything from Nahalem to Broadwell still has some IME modules left, which are ROMP and BUP. After Broadwell, which is Skylake to now, there are at least 4 modules, rbe, kernel, syslib and bup. While Purism has been able to neutralize most of the IME, their progress is currently locked by me_cleaner, and they have been unable to successfully neutralize the IME in their lastest product offerings, which are the Librem Mini (v2) and Librem 14 (v1).
You can learn more about the me_cleaner tool used by Purism in the URL below.
If Purism is able to completely reverse engineer the remaining IME modules, I would consider purchasing a hypothetical motherboard bundle from Purism, but as it stands, the risk of a theoretical attack using the remaining IME is too great to justify the expense.
This is assuming that Purism still intends on using x86.